A monochord is a one-stringed instrument that was used throughout the middle ages as a tool to explore the relationship between mathematics and sound. For instance, if a string is divided in half, it produces a tone one octave higher than the open string. Other ratios such as 3:2 (fifth), 4:3 (fourth), and 9:8 (whole tone) can be found on the monochord, at once clearly visible and audible. A section on how to ‘divide’ the monochord was a standard inclusion in medieval theoretical writings on music. In many instances, theorists described tuning systems to meet the priorities of the music of their time.

Instrument composed of a resonance box on which is a single string, which rest on two mobile pegs.( the greek theorists used the instrument for the study and calculation of the sound vibration.

In the middle ages it was used to test the differences of tuning fork and the intervals, always with the sane name, to share of the S. XVI, the monochord came to have fourteen strigs, the clavichord and the trumscheit belong to the same family