To provide a nut and a nut auxiliary tool and a saddle which can be readjusted easily without adding new processing to the main body of an musical instrument even when the size of a string to be used is changed.

Spacing between a nut or a nut auxiliary tool and a 1st fret is optimized based on a calculated value as shown in the figure 30. Moreover, spacing between a saddle piece and a 12th fret is optimized based on a calculated value, and, furthermore, spacing between a saddle piece of a saddle piece and the 12th fret at the time of using the nut or the nut auxiliary tool is optimized based on a calculated value. Thus, the pitch of a sound which is obtained by pressing down a string at the 1st fret is made the pitch of a sound which is higher than the pitch of a sound which is sounded by an open string by semitone and the pitch of a sound which is obtained by pressing the string at the 12th fret is made the pitch of a sound which is higher than the pitch of the sound which is sounded by the open string by one octave. By constituting a fret stringed instrument like this, ascension of pitches of sounds of the 1st fret and the 12th fret in which ascension of pitches of sounds by influence of a string to be used, an open string and the height difference of frets is remarkable is eased.