The present invention relates to an interconnectable building system and particularly, though not exclusively, to an interconnectable toy building system comprising pluralities of building elements, connectors, adapters, and linear elements for connecting building elements together and connecting accessories to the building elements . Toy construction sets, comprising interconnectable blocks are well known, a common example being simple interlocking modular bricks of different shapes, sizes and colours formed from a resilient plastic material. The simplest examples of interlocking modular bricks take the form of cuboidal building elements comprising complementary arrays of circular protrusions and recesses on their upper and lower surfaces. Toy construction sets of this type are significantly constrained insofar as connections are possible in only one

direction - for example vertically - and so are typically only suitable for young children.

More complex toy construction sets - such as shown in US 5,964,635 (Interlego AG) - are additionally provided with a series of

cylindrical cavities extending laterally between opposite side surfaces of each cuboidal building element. Aligned cylindrical cavities of different building elements may then be connected together side by side via separate cylindrical connector pegs. Toy construction sets of this type are less constrained than the simple interlocking modular bricks mentioned above since connectivity is possible in two mutually orthogonal directions. However,

limitations still remain in terms of the flexibility and

intuitiveness of such existing toy construction sets.

The inventor of the present invention has identified a need for an improved set of interconnectable toy building elements which overcomes, or at least ameliorates one or more of the aforementioned problems, e.g. by providing enhanced connectivity between building elements and a more intuitive and enjoyable experience for users. According to the invention, there is provided an interconnectable building system comprising:

a plurality of building elements, at least one connector for connecting two or more building elements or connecting a building element and another component, an adapter for adapting the

connectivity of a building element and a linear element/member/axle for connecting to the adapter,

wherein the building element comprises a substantially hollow body having a plurality of holes therein for receiving a connector or a linear element, and wherein the body is arranged to receive the adapter so that the linear element engages the building element via the adapter.

Preferably the adapter is removably insertable in a cavity within the body of the building element. The adapter may be resiliently deformable and may have a port/hole/throughbore/cavity for receiving the linear element. The port is preferably shaped to correspond with a sectional profile of the linear element. In a preferred arrangement the interconnectable building system comprises a building element having at least four planar faces having a hole in each of the four faces, the hole extending normally from one face through the centre and through the opposite face forming a throughbore and including a counterbore and also

optionally including an enlarged cavity in the centre of the building element in way of the throughbore the counterbore having a diameter greater than that of the hole and an annular base and the cavity having an internal wall. There may be a resiliently deformable connecting element with at least a part having an essentially cylindrical form said part having a diameter to fit the hole / throughbore in the building element and the connector preferably is provided with one / more of a collar and / or a lip and / or a part engageable with the counterbore in the building element.

Preferably a resiliently deformable adapter element provides the building element with modified connectivity characteristics said adapter comprising. The adaptor preferably includes a resiliently deformable body/ies shaped and dimensioned for removable insertion via the hole and throughbore into the cavity and having upper and / or lower extensions locatable in the throughbore. The adaptor may include an outer side surface of the body/ies profiled to engage with at least a part of the internal wall of the cavity and / or the throughbore; and may comprise an inner surface of the body/ies profiled for the linear element to be removably fitted into the inner surface profile after said adapter has been fitted into the cavity.

In a preferred arrangement the system includes a linear element profiled to fit in an appropriately formed aperture in the body of the resiliently deformable adapter. The resiliently deformable connecting element may be removably inserted via the hole into the throughbore in the building element such that one / more of the collar or the lip or the part engageable with the counterbore will positively locate the connecting element in the building element and allow the connection of another building element to the connecting element such that the two building elements are rigidly connected together. The adapter may be fitted into the cavity and the linear element fitted into the inner profiled surface of the adapter to form an axle or connection means extending normally from a face(s) of the building element. All the elements of the interconnectable building system described herein and other elements not described but embodying the features

described may comprise a kit of parts any selection of which may be used for a particular assembly and subsequent dis-assembly . The end faces of the building element may be either curved or flat.

The flat end of the building element may be provided with a hole extending into the central cavity/cavities and optionally also with a counterbore.

The counterbore in the building element may be an enlargement of the throughbore and may extend a predetermined distance from the planar face and both throughbore and counterbore are essentially circular in form. The throughbore and / or counterbore in the building element may have a form other than that of a hollow, right cylinder. Each of the four planar faces of the building element may be aligned orthogonally to the two planar faces that are adjacent.

The building element may be a right orthogonal cuboid with six planar faces, i.e. four contiguous sides and two ends, with the bores and counterbores normal to the planar faces.

The resiliently deformable connecting element may have a hollow central bore. The resiliently deformable connecting element may be provided with a slot in way of the lip or engageable part so that it may be deformed to insert it into the hole and pushed through said hole and, once through, expand to its former dimension to grip the base of the counterbore in the diametrically opposite face.

The axial length of the collar and its location along the axial length of the resiliently deformable connecting element may

determine how closely the abutting faces of interconnected building elements are, i.e. whether in direct face-to-face contact or spaced apart .

The hollow body of the resiliently deformable adapter may be provided with a means to engage with and removably hold the / a part of the linear element or to allow it rotate within its engagement.

The internal wall of the central cavity of the building element may have the profile of a sphere or part of a sphere.

The internal wall of the central cavity may have the profile of part of a cylinder or the intersection of two or more part-cylinders.

The internal wall of the central cavity may have the profile of the intersection of two or three mutually orthogonal part cylinders. A plurality of holes and counterbores may be provided along the longitudinal planar faces of the building element.

The central cavity within a building element may communicate directly with the adjacent one(s) to form s one or more contiguous, longitudinal cavity/cavities.

The longitudinal cross-section of the building element may be X- shaped or maybe I-shaped except where it is interrupted by other features such as a hole, counterbore or cavity.

The manufacture of the building elements may include any / or combinations of machining from solid, die casting, injection moulding and extrusion and / or surface finishing by either

electrochemical polishing (electropolishing) , chemical polishing (chemical deburring} or a combination of both processes.

A building system element may be manufactured from more than one material, the additional material (s) may be used to provide one or more reinforcing members within the primary material of the system element .

The surfaces of the reinforcing elements may be treated to improve bonding with the substrate of the building element.

The adapter may be provided either as a single item fittable into the cavity via the hole / throughbore or as two matched halves and fitted into the same cavity via either the hole in a single side or the two holes forming the throughbore in opposite sides of the building element.

The adapter may include bifurcated parts to facilitate deformation during removable insertion and / or elements to engage with the counterbore ( s ) .

The adapter extensio (s) within the counterbore (s) may each be either profiled to be flush with the insertion and / or opposite planar faces of the building element or are extended beyond said planar faces, said adapter extension (s) may also be modified so that an extension element is fittable thereto to enable a rotational adjustment to be provided to the adapter.

The inner faces of the adapter may be shaped to permit fitting of a linear element having either / both a cruciform or a circular cross sectio .

The dimensions of the adapter's body(ies), the elements that engage with the throughbores, the nature of its deformable material and overall shape may be such that it forms a sprung frictional fit in the cavity and / or throughbores of the building element, to reduce any free play between it and the building element.

In a preferred application of the building system of the invention, the building elements may be of metal, rigid plastic or fibre reinforced plastic or a combination of these materials and the deformable connectors and adapters of a good quality resilient elastomer. The choice of materials may vary according to the particular applications. The system may include appropriate variations of the four basic elements and appropriate variations of each, e.g. different lengths of building elements and different lengths of connectors. The counterbores may include countersinking and conical or frustoconical forms with matching ends to the collars. As the building elements are normally cuboid, it is easy to fit them together via the resiliently deformable connectors and disconnect and / or modify the arrangement so that new products can be created, e.g. as prototypes, and subsequently tested and

developed prior to marketing. The building system is intended to facilitate the designing of prototypes and developing them. Consequently, it is impossible to define every size, shape and interconnector that might ever be required. Though cuboid, orthogonal building elements are taught, the skilled person will appreciate that, in some cases, unique forms will be required, e.g. having hexagonal sections (not shown) as opposed to the square sections taught. These may be produced as ^specials' using the same connecting technology, as taught herein, but with appropriate dimensions, e.g. shorter or longer cylindrical lengths. Similarly, the building elements may have X or I-shaped cross sections. Where the building elements are made of plastic, they may be reinforced, where required.

In a preferred arrangement, manufacture of the building elements utilises an extruded tube with an outer profile that is principally square and an inner profile that is principally either circular or square, principally being taken to mean including deviations from the basic shape, for example principally square would include a square with radiused corners.

The inner surfaces of the hollow bodies of the adapters may be profiled to accommodate cruciform, cylindrical or other shapes of axles / linear elements but, as before, may be especially profiled to allow uniquely shaped items to be fitted in order to meet the particular requirements of prototype developments. This will allow novel products to be produced where rotational freedoms may, or may not, be required and also accommodate sliding or frictional fits.

Preferably, the surface profile of the cavity is spherical or part spherical but other forms are equally possible. For most

applications, the building elements will be four-sided orthogonal members with two planar ends, normal to the sides so that

connectivity will be triadic, i.e. in the three mutually orthogonal axes. The cross section of the building elements may be either square or X-shaped or I-shaped. Where required, the building elements may include reinforcing members, i.e. they may use more than one material of construction.

Where appropriate, the adapters and connecting elements are provided with means to turn (adjust) them in the central cavities to align them for fitting linear elements. The use of both circular and other cross sectioned linear elements increases the scope of the invention when designing prototypes or developing them. It will be appreciated that each connector is shaped and dimensioned to facilitate the removable location of each bifurcated end within a building element. In one preference, the length of each connector is selected to be twice that of the axial width of a building element (taking into account manufacturing tolerances), including the sum of the counterbore depths. Accordingly, when connected with a building element, the / one bifurcated end extends axially through its cavity beyond its centre point to the opposite counterbore. The central annular collar of the connector has an axial width

equivalent to twice the depth of each counterbore; and a diameter which is closely matched to that of each counterbore (taking into account manufacturing tolerances) . Therefore, when the connector is fully inserted within a cavity, 50% of the axial width of the central annular collar is located within its counterbore.

Similarly, a second building element may be fitted to the projecting part of the connector.

Preferably, the extremities of each bifurcated end most distal to said central annular collar are provided with an outwardly

protruding annular lip. It will be appreciated that the axial length and radial width of each outwardly protruding annular lip is such that it can be wholly accommodated within a counterbore in a resilient snap-fit fashion. The invention also includes a kit of parts comprising an

interconnectable building system including:

a plurality of building elements, at least one connector for connecting two or more building elements or connecting a building element and another component, an adapter for adapting the

connectivity of a building element and a linear member for

connecting to the adapter, wherein the building element comprises a substantially hollow body having a plurality of holes therein for receiving a connector or a linear element, and wherein the body is arranged to receive the adapter so that the linear element engages the building element via the adapter.

The invention also includes a kit of parts comprising an

interconnectable building system, including at least one building element comprising a substantially hollow body having a plurality of holes therein for receiving a connector or a linear element, and wherein the body is arranged to receive an adapter for adapting the connectivity of the building element. The invention also includes a kit of parts comprising an

interconnectable building system, including at least one adaptor £0 adapting the connectivity of a building element by location of the adaptor inside a body of a building element.

The invention also includes method of assembly of an

interconnectable building structure, the method comprising:

connecting two or more building elements or connecting a building element and another component and adapting the connectivity of a building element by inserting the adapter into a body of the building element so that a linear element engages the building element via the adapter.

For a clearer understanding of the invention and to show how it may be put into effect, reference will now be made, by way of example only, to the accompanying drawings in which:

Fig. 1 shows a prior art building element design illustrated via a set of mutually perpendicular orthographic projections and an isometric projection;

Fig. 2 shows an isometric projection and sectional end view of a prior art building element;

Fig. 3 shows an isometric projection and mid-line sectional side view of a prior art building element;

Fig. 4 shows a triadic-connectivity building element design

according to invention, illustrated via a set of mutually perpendicular orthographic projections and an isometric proj ection;

Fig. 5 shows an isometric and sectional end view of the building element of Fig. 4;

Fig. 6 shows an isometric and mid-line sectional side view of the building element of Fig. 4; Fig. 7 shows a dyadic-connectivity building element design according to the invention, illustrated via a set of mutually

perpendicular orthographic projections and an isometric projection;

Fig. 8 shows an isometric and sectional end view of the building element of Fig. 7;

Fig. 9 shows an isometric and mid-line sectional side view of the building element of Fig. 7;

Fig. 10 shows a set of three building elements and three connectors according to the invention, ready to be assembled;

Fig. 11 shows the three building elements of Fig. 10 in assembled form;

Fig. 12 shows a set of views of a connector engaged within a

spherically smoothed cavity of a building element;

Fig. 13 shows a set of views of a connector engaged within a non- smoothed cavity of a building element;

Fig. 14 shows examples of differently shaped cuboidal building

elements ;

Fig. 15 shows one example of a differently shaped non-cuboidal

building element;

Fig. 16 shows a reduced material volume, sculpted building element with an X-core longitudinal brace;

Fig. 17 shows a reduced material volume, sculpted building element with an I-core longitudinal brace;

Fig. 18 shows a reduced material volume building element (not

forming part of the present invention) with an inner square tubular longitudinal profile; Fig. 19 shows a single unit triadic building element and illustrates the areas therein subject to the most stress;

Fig. 20 shows a four unit dyadic building element and illustrates how this may be reinforced via a set of four high strength rods ;

Fig. 21 shows a four unit dyadic sculpted building element and

illustrates how this may be reinforced via a set of four high strength rods;

Fig. 22 shows a four unit dyadic sculpted building element and

illustrates how this may be reinforced via a complex cage comprising intersecting high strength rods and tori;

Fig. 23 shows a three unit building element, and an adapter and

corresponding axle according to the invention; all prior to assembly; Fig. 24 shows the adapter in-situ within the cavity of the building element of Fig. 23;

Fig. 25 shows a building element, adapter and axle of Fig. 23 in assembled form;

Fig. 26 shows one example of an adapter according to the invention illustrated via a set of mutually perpendicular orthographic projections and an isometric projection; Fig. 27 shows a first alternative design of an adapter according to the invention;

Fig. 28 shows a second alternative design of an adapter according to the invention illustrated via an orthographic projection of the end and an isometric projection;

Fig. 29 shows an adapter tool ready for engagement with an adapter according to the invention; Fig. 30 shows the adapter tool of Fig. 29 engaged with an adapter according to the invention;

Fig. 31 shows a second type of adapter illustrated via a set of

mutually perpendicular orthographic projections and an

isometric projection;

Fig. 32 shows an alternative design for the second type of adapter illustrated via an orthographic projection and an isometric projection;

Fig. 33 shows a second type of adapter tool ready for engagement with the second type of adapter according to Figs. 31 or 32;

Fig. 34 shows the second type of adapter tool engaged with the

second type of adapter according to Figs. 31 or 32;

Fig. 35 illustrates the first step of an assembly sequence

illustrated via an isometric projection, sectional view and plan view, wherein the second type of adapter of Figs. 31 and 32 is ready to be inserted into single unit dyadic building element;

Fig. 36 illustrates the second step of the assembly sequence whereby the adapter is inserted, but not yet snapped into, its final assembly position within the cavity;

Fig. 37 illustrates the third step of the assembly sequence whereby the adapter is snapped into its final assembly position within the upper half of the cavity;

Fig. 38 illustrates the fourth step of the assembly sequence whereby a second adapter of the second type is ready to be inserted into the remaining lower half of the cavity;

Fig. 39 illustrates the fifth step of the assembly sequence whereby the second adapter of the second type is inserted, but not yet snapped into, its final assembly position within the cavity; Fig. 40 illustrates the sixth step of the assembly sequence whereby the second adapter of the second type is snapped into its final assembly position within the lower half of the cavity; Fig. 41 shows a third type of adapter illustrated via a set of

mutually perpendicular orthographic projections and isometric projections ;

Fig. 42 shows a single unit dyadic building element and a pair of adapters of Fig. 41 prior to their assembly; and

Fig. 43 shows single unit dyadic building element and a pair of

adapters shown in Fig. 42 in their assembled form, this being illustrated via an isometric projection, sectional view and plan view.

In the following description, the same reference numeral is used for the same part in different Figures or for different parts fulfilling an identical function.

Fig. 1 shows a prior art interconnectable toy building element 1 having a linear sequence of three connector openings 2 on opposite side surfaces thereof. As best shown in Figs. 2 and 3, the openings 2 are at the opposite ends of a uniform cylindrical cavity 3. The cylindrical cavity 3 enlarges diametrically in a stepwise manner into axially aligned counterbores 4 formed in each side surface of the building element 1.

One example of an interconnectable toy building element 5 according to the present invention is shown in Fig. 4. Building element 5 has a single circular connector opening 6 on each of its two end surfaces; and linear sequences of four circular connector openings 7, 8 spaced longitudinally at regular intervals 9, 10 on its respective side and upper surfaces. Such an arrangement can be defined as having triadic connectivity i.e. connectivity between different building elements 5 is possible in three mutually

orthogonal directions via connector openings 6, 7, 8. Each

connector opening 6, 1, 8 enlarges diametrically in a stepwise manner into axially aligned counterbores 12 (Fig. 6) formed in ea surface of the building element 5.

Connector openings 7, 8, 9, 10 on the respective sides of the building element 5 may be grouped into notional pairs 7, 10 and 8, 9 whereby each pair is in axial alignment. The mutually orthogonal central axes of each notional pair 7, 10 and 8, 9 intersect at the centre of a cavity 11 (Fig. 5) which is common to each set of notional connector opening pairs 1, 10 and 8, 9. The first and second (and the third and fourth) cavities 11 of the linear sequence are contiguous via adjoining annular grooves 13. The diameter of each groove 13 is matched to that of each counterbore 12. The width of each groove 13 as measured in the longitudinal direction is equivalent to twice the axial depth of each counterbore 12 (taking into account manufacturing tolerances) . A solid internal wall 14 separates the second and third cavities 11.

In the particular example illustrated in Figs. 4 to 6 the walls of the cavity 11 have a spherical surface profile. Consequently, and as best shown in the sectional view of Fig. 6, and in Figs. 12 and 13, each cavity 11 has a non-uniform diameter when measured

perpendicularly through different points along the notional central axes extending between notional connector opening pairs 7, 10 and 8, 9. In particular, the diameter of the cavity 11 is at its minimal extent proximate to the base of each counterbore 12 (i.e.

corresponding to the position of the annular support surfaces 26, 27 of Figs. 12 and 13). The diameter of the cavity 11 is at its maximal extent at the axial mid-point between each notional

connector opening pair 7, 10 and 8, 9.

Another example of an interconnectable toy building element 15 according to the present invention is shown in Fig. 7. It differs in structure from the building element 5 of Figs 4 to 6 in three main respects. Firstly, building element 15 has no connector openings on its two end surfaces. Such an arrangement can therefore be defined as having dyadic rather than triadic connectivity, i.e. connectivity between different building elements 15 is possible in only two mutually orthogonal directions via connector openings 16, 17. Secondly, the walls of the cavity 18 are formed via the intersection of two mutually orthogonal cylindrical bores.

Accordingly, in contrast to the spherical surface profile described above, the surface profile {as best shown in Fig. 8} of each cavity 18 is segmented. This type of non-spherical surface profile takes the form of a bicylinder. Thirdly, each cavity 18 is isolated from its adjacent cavity by intervening solid walls {as best shown in Fig. 9). Those differences aside, the building element 15 of Figs. 7 to 9 is otherwise equivalent to that of Figs 4 to 6. Fig. 10 shows a set of three triadic building elements 20, 21, 22 - of the type described above with reference to Figs. 4 to 6 - prior to being releasably assembled together via connectors 19. It will be appreciated that there are a multiplicity of possible

interconnections that may be made between the building elements 20, 21, 22. Although the particular building elements shown in Fig. 10 comprise three, four and five linear units respectively, it will be appreciated that other sizes are possible, e.g. single units, double units and six or more linear units. Indeed, more complex building elements 29, 30, 31, 32 having different three-dimensional

configurations comprising combinations of cuboidal building elements are also possible, examples of which are shown in Fig. 14. An example of a building element 33 having non-cuboidal ends 34 is also shown in Fig. 15. The connectors 19 (Fig. 10} are of a known design and described briefly below. Fig. 11 shows the set of three triadic building elements 20, 21, 22 once they are assembled together using the connectors 19. Fig. 12 shows a set of views of a connector 19 engaged within a spherically smoothed cavity 11 of a triadic-connectivity building element 5 of the type described above with reference to Figs. 4 to 6, 10 and 11. Each connector 19 is tubular, and provided with resiliently flexible bifurcated ends 23 defined by relief cuts 24 formed in the tubular walls. Opposing ends of each connector 19 are separated by an outwardly protruding central annular collar 25. It will be noted how throughbores 26 (Fig. 12) and 27 (Fig. 13} give support and location to connector 19 across the full width of building elements 5 (Fig. 12) , and 15 (Fig. 13) , by engaging with the connector at each side of the building element.

In use, the initial insertion of a connector 19 into a connector opening of building element 5 causes an outwardly protruding annular lip formed around the distal edge of each bifurcated end 23 to abut against the base of the counterbore 12. A continued manual

insertion force applied in the axial direction allows the bifurcated ends 23 to flex radially inwards and bypass narrow throughbore 26 of constant width at the base of the counterbore 12. This is made possible due to the presence of relief cuts 24 in the tubular walls of the connector 19; and because each annular lip is provided with a rounded cam surface profile allowing each lip to ride radially inwardly along the base of the counterbore 12.

A continuing manual insertion force applied in the axial direction forces the bifurcated ends 23 through the narrow throughbore 26 associated with the connector opening at the opposite side of the building element 5. Once the bifurcated ends 23 enter the opposite counterbore 12 they "expand radially outwards in a snap fashion. In doing so, the protruding annular lips are seated behind the base of the counterbore 12 thereby resisting retraction of the connector in the reverse axial direction. Meanwhile, the central annular collar 25 is partially seated in the opposing counterbore 12, thus

preventing further axial insertion of the connector 19,

It will be appreciated that, when fully in~situ, the connector 19 is annularly supported by the narrowest internal parts of the building element 5, i.e. by the annular support surfaces of throughbores 26 proximate at the base of each opposing counterbore 12. The central portion of each connector 19 between the annular collar 25 and the bifurcated ends 23 is unsupported due to the spherical concave surface profile of the cavity 11. Fig. 13 shows a set of views of a connector 19 engaged within a non- smoothed cavity 28 of a dyadic-connectivity building element 15 of the type described above with reference to Figs. 7 to 9. The process for insertion of a connector 19 into building element 15 is the same as that described above with respect to the building element 5 of Fig. 12. The only minor difference arises due to the non-spherical concave surface profile of the cavity 28. As a consequence of this non-spherical surface profile, each annular support surface (hole / throughbore) 27 has a variable width around its circumference. However, the central portion of each connector 19 between the annular collar 25 and the bifurcated ends 23 remains unsupported due to the concave non-spherical surface profile of the cavity 28. Fig. 14 shows examples of different shapes 29, 30, 31 & 32 that building elements may take beyond the basic cuboidal forms 5 &15 (Figs. 4 & 7) already illustrated. These specific shapes are intended as generic examples only; clearly other shapes with more or fewer connector holes are possible, for example the "plate" element 32 is drawn as having a hole grid of 3x2 but could equally well be designed as 3x3 or 3x4. Further, the elements shown are all triadic-connectivity, but could equally well be designed as dyadic- connectivity . Fig. 15 shows a non-cuboidal building element with part cylindrical ends. This design confers the benefit of the element still being able to rotate about the axis of the cylinder that defines the end surface, whilst said end is in close proximity to another building element .

Fig. 16 shows a dyadic building element 35 provided with an internal structural framework interposed between each counterbore / cavity. The framework adopts an X-shape 36 when viewed in section. Material has been selectively removed from the outer side surfaces of the building element 35 between each counterbore / cavity to provide concavities 37. Material has also been removed from both end surfaces to provide a sculpted appearance which adopts the shape of a- part of a bicylinder 38. It will be appreciated that this structure provides a substantially uniform wall thickness at all parts of the building element 35, and hence a marked reduction in overall material volume. This type of structure is particularly suited to injection moulding or die casting techniques and so aids the manufacturing process. Similarly, Fig. 17 shows a dyadic building element 39 provided with an internal structural framework interposed between each counterbore / cavity. The framework adopts an I-shape 40, when viewed in section. Material has been selectively removed from the outer side surfaces of the building element 39 between each counterbore / cavity to provide concavities 41. As described above, material has also been removed from both end surfaces to provide a sculpted appearance which adopts the shape of a- part of a bicylinder 38.

This structure provides the same benefits as mentioned above with respect to Fig. 16.

Fig. 18 shows a dyadic building element 42 provided with a tubular structural framework 43 surrounding a single cavity extending along its full length. Whilst a square tube is illustrated, other cross- sectional shapes are possible, including circles, and squares with rounded external corners. The internal junctions between different walls may optionally be rounded or chamfered 44 to facilitate easier insertion of a connector 19. This type of structure is particularly suited to manufacture by machining techniques, e.g. from a custom extruded aluminium alloy tube. An extruded square tube with a suitable circular internal profile could be used as the basis for machining triadic-connectivity building elements much more

efficiently than starting with solid square bar. Such building elements once finished would appear exactly the same those shown in Figs. 4-6, except all the internal walls 14 would be provided with longitudinally extending cylindrical holes.

In order to ensure that connector elements and adapter elements may be easily inserted into a building element and also to avoid any unnecessary wear occurring to them, the surfaces of the building element need to be smooth. This could be accomplished using

electrochemical or chemical polishing.

Fig. 19 shows a single unit triadic building element. While in use, the areas of material subject to the greatest mechanical stress are those indicted by reference numerals 45, 46, i.e. proximate to the side wall and base of each counterbore 12. Such mechanical stresses demand that the building elements are manufactured from a material having suitable strength characteristics. In one example, the entire building element may be formed from a single homogenous high strength material such as aluminium alloy; or an engineering grade plastics material. However, composite materials may also represent a good compromise between strength and cost so an alternative option would include a glass-filled polymer containing either random fibre reinforcement or aligned fibres.

Fig. 20 illustrated the inclusion of four high strength linear elements in the form of rods 47 used to reinforce a building element 15. The rods 47 may be formed from a metallic wire and need not necessarily be circular, as shown. The rods 47 extend through the full length of the building element 15 and pass through the

innermost high stress regions 45 described above with reference to Fig. 19 . Rods 47 may additionally or alternatively pass through the outermost high stress regions 46. The inclusion of such high strength rods 47 may allow the bulk of the building element 15 to be formed from a relatively low strength material, and hence lower cost. The rods 47 may be inserted into a mould prior to an

injection moulding process. The external surface of each rod 47 may be mechanically or chemically roughened to improve their bond strength .

Alternative combinations of strengthening rods 47, 50 and

intersecting tori 50 are illustrated in Figs. 21 and 22.

One example of an adapter 51 for modifying the connectivity

characteristics of an interconnectable building element 52 is illustrated in Fig. 26. The adapter 51 is in the form of a

resiliently deformable hollow body shaped and dimensioned for removable insertion into a cavity. The adapter 51 is provided with resiliently deformable bulbous side surfaces 56. The top, bottom, front and rear outer surfaces are substantially planar. A cross- shaped passage 54 extends through the adapter 51 between its front and rear surfaces. Smaller axially aligned cylindrical bores extend through the adapter 51 between its top and bottom outer surfaces and are contiguous with the cross-shaped opening. The cylindrical bores are contained within upper and lower extensions of the adapter 51. In use, the initial insertion of an adapter 51 into an upper connector opening of building element 52 (shown in Figs. 23-25) causes its bulbous side surfaces 56 to abut against the base of counterbore 12. A continued manual insertion force applied in the vertical axial direction forces the bulbous side surfaces to flex radially inwards and bypass narrow throughbore surface 26 (Fig, 12) at the base of the counterbore 12. Once the bulbous side surfaces 56 enter the cavity 11, they expand radially outwards so as to be seated behind the base of each throughbore 26 into counterbore 12, where they closely fit with the internal surface of cavity 11 (Fig. 12), thereby resisting further movement of the adapter 51 in either axial direction. The upper and lower extensions of the adapter 51 which contain the axially aligned cylindrical bores are part- circular when viewed in section (Fig. 26) .

Once the bulbous side surfaces 56 of the adapter 51 are located within the cavity (see Fig. 24) , their resiliently deformable nature closely fits against the internal spherical surfaces of the cavity 11, accommodating any manufacturing tolerances. Similarly, the part-circular outer surfaces of the adapter's upper and lower extensions closely fib against the aforementioned throughbores 26 associated with the connector openings at the opposite upper and lower surfaces of the building element 52. The upper and lower extensions do not extend outside counterbore 12. The adapter 51 is therefore constrained against lateral movement relative to the vertical central axis. Rotational movement of the adapter 51 is possible relative to the vertical central axis and indeed is required to ensure that the front and rear surfaces face the front and rear cavities; and that the central axis of the cross-shaped passage 54 is coaxial with the corresponding central axis of the cavity .

In short, the adapter 51 has the fundamental characteristic that it is securely retained within the cavity by one opposing pair of connector openings (i.e. the top and bottom connector openings as shown in Figs. 23 to 25) whilst presenting a modified opening of different cross-sectional size and/or shape in a mutually orthogonal opposing pair of connector openings (i.e. the front and rear connector openings as shown in Figs. 23 to 25). Once the adapter 51 is in-situ and properly rotationally aligned within the cavity, an axle element 53 (of known design), e.g. of a complementary cruciform shape, may be introduced into the cross- shaped passage 54 (see Fig. 25) . Depending on the precise

dimensions and shapes of inner walls 55 defining passage 54, said axle element 53 may either be provided with a friction fit in the adapter 51 or a loose sliding fit. It will be appreciated that the axle element 53 is incapable of rotational movement relative to the adapter 51 and hence the building element 52. Though axle 53 is not rotatable, passage 54 may equally be profiled to accept a

cylindrical axle (not shown) . Where movement is required between axle 53 and passage 54, lubrication, such as talc or silicone substances, may be provided.

An alternative example of an adapter 57 for modifying the

connectivity characteristics of an interconnectable building element 52 is illustrated in Fig. 27. Whilst it is functionally equivalent to the adapter 51 described above with respect to Fig. 26, its upper extension is bifurcated such that the bifurcations may be each compressed towards each other within the intervening gap 58. Such a structure reduces the amount of force required to compress the bulbous sides 56 and therefore makes it relatively easier to insert and remove the adapter 57 from a cavity of a building element 52.

A further modification is shown in Fig. 28 whereby an adapter 59 - also functionally equivalent to those of Figs. 26 and 27 - is adapted such that both its upper and lower extensions are bifurcated and compressible into intervening gaps 61. Two small protrusions 60 are provided on the outer curved surface of each bifurcation to provide frictional engagement between them and the previously described narrow annular support surface 26 (hole) associated with each connector opening. This arrangement may minimise or eliminate any free play between the adapter 59 and the building element.

A manually graspable tool 62 is provided with a socket 63 for engaging the upper or lower extension 64 of an adapter 51, 57, 59. The tool 62 (shown in Figs. 29 and 30) is capable of engaging an adapter 51, 57, 59 whilst it is in-situ within a cavity of a building element, e.g. as shown in Fig. 24. The tool 62 may be used to rotate an adapter 51, 57, 59 to align its cross-shaped passage ready for receiving an axle element 53.

A yet further modification is shown in Figs. 31 to 40 whereby two adapter parts 65 (see Figs. 31 and 35 to 37) and 76 (see Fig. 38) cooperate to form a completed adapter combination (see Fig. 40) which is functionally equivalent to the one-piece adapters 51, 57, 59 described above with respect to Figs. 26 to 28. Adapter part 65 has an overall circular profile 66 to permit it to slide into a cavity as shown in Fig. 37; and is provided with flanges 67 that help retain it therein. When inserted into the cavity of a building element 75 the spring slots 68 permit the flanges 67 to flex inwards allowing the flanges to be pushed through the cavity from the inside. Upon reaching the far side of the cavity the flanges 67 spring outwards again, providing a snap effect that engages within the counterbore of the cavity and prevents the adapter part 65 from easily sliding back out. An axle channel 69 is formed in the adapter part 65 that fits over one side of an axle element 53 (see Fig. 23) . The other adapter part 76 engages the other side of the axle element.

As shown in Fig. 32, a modified adapter part 70 is provided with a set of small smooth protrusions 71 that lightly contact the narrow annular support surface 26 associated with each connector opening. In so doing they provide a firm installation of the adapter part 70 in the building element without any free play. The axle channel 69 is also provided with a set of small smooth protrusions 72 that provide a tight sprung friction grip on an axle element 53 that has been inserted into the channel.

A second type of adapter tool 73 {Figs. 33, 34) is provided for manipulating the adapter parts 65, 70 into position. The tool 73 has a protrusion 74 onto which the second type of cavity adapter 65 or 70 can be temporarily mounted. Once assembled (see Fig. 34), the tool is used to insert the adapter into a building element cavity and snap it into its final position. Figs 35 to 40 show how a pair of adapter parts 65, 76 are

sequentially inserted into a cavity and snapped into their final positions. In Fig. 35 an upper adapter part 65 is aligned ready to slide into a building element 75. In Fig. 36, the upper adapter part 65 has been slid into the building element 75. This is made possible because of its outer circular profile 66 that matches the profile of the cavity of the building element 75. In Fig. 37, the upper adapter part 65 has been displaced vertically upwards, snapping it into its final assembled position. It is held in place by its flanges 67 that have sprung outwards into the counterbore 12 of the upper cavity. The upper adapter part 65 axle channel 69 is now correctly aligned within the side cavity.

In Fig. 38, a lower adapter part 76 is aligned ready to slide into the building element 75 below upper adapter part 65. In Fig. 39, the lower adapter part 76 has been slid into the building element 75. The plan view shows how the upper adapter part 65 and lower adapter part 76 fit alongside each other within the narrow

throughbore 26 associated with the connector opening. In Fig. 40, the lower adapter part 76 has been displaced vertically downwards, snapping it into its final assembled position. The axle channels of the upper adapter part 65 and lower adapter part 76 form a cross shaped through socket 77, ready to receive an axle element 53. Fig. 41 shows a yet further adapter part design 78 having four flanges 79 that retain it in a building element cavity. When inserted into a cavity, the spring slots 80 permit the flanges 79 to flex inwards allowing the flanges to be pushed through the cavity. Upon reaching the inside of the cavity the flanges 79 spring outwards again, providing a snap effect that stops the adapter part from easily sliding back out. A second flange 81 is provided which prevents it from being pushed further into the cavity beyond the base of the counterbore 12. An axle channel 82 is formed in the adapter part 78 that fits over one side of an axle element 53 (see Fig. 23) . A socket 83 is provided to allow the adapter part 78 to be rotated for alignment purposes whilst installed in a building element. A hexagonal socket would be suitable for this purpose. The adapter part 78 is designed to be utilised in pairs, thus engaging both sides of an axle element 53 simultaneously. Fig. 42 shows top and bottom adapter parts 83, 84 ready to be inserted into opposing openings of an cavity in a building element 75. Fig. 43 shows both the top 83 and bottom 84 adapter parts assembled in the building element 75, They are held in place by their flanges 79 that have sprung outwards and engaged with

throughbore 27 (Fig. 13) of the cavity that is mutually

perpendicular to the counterbore through which it was inserted. The axle channels of the top 83 and ^" bottom 84 adapter parts form a cross shaped through socket 85, ready to receive an axle element 53.

Although particular embodiments of the invention have been disclosed herein in detail, this has been done by way of example and for the purposes of illustration only. The aforementioned embodiments are not intended to be limiting with respect to the scope of the appended claims. Indeed, it is contemplated by the inventor that various substitutions, alterations, and modifications may be made to the invention without departing from the claim scope. As the system of the invention is intended to facilitate prototyping and

development of new products and processes and their subsequent optimisation, new constructional elements are certain to be required for bespoke applications and will embody the principles taught herein . For example, it should be noted that many of the drawings depict sharp external and internal corners in order to promote clarity. However, those corners may instead be rounded, filleted or chamfered according to specific requirements. Furthermore, although the foregoing description describes the embodiment a building system in the form of a toy, the system could equally be intended for other purposes such as engineering prototypes. It should also be

understood that the connectivity of a building element, be it triadic or dyadic, is not constrained by the form that the surface profile of the cavity takes, for example a dyadic-connectivity building element with a spherically smoothed cavity is feasible.