A monochord is a musical and scientific instrument dating back to ancient human history. Traditionally, it is simply a single string that is stretched over a sound box and fixed at either end with pegs. When the string is plucked, the side of the sound box that is in contact with the string pegs vibrates along with the string, thereby moving the air inside. This increases the amplitude of the tone produced before it is projected outward.

A moveable fret is often used to change the length of the portion of the string that is vibrating, and thereby change the frequency of the note being produced.

The monochord can be used to illustrate physical and mathematical properties of music; particularly the relationship of frequency and pitch. For example, when a monochord's string is open it will produce a given musical note. When the length of the string is halved, the string producing a note of twice the frequency of the original, sounding one octave higher. Half of this length will produce another octave higher, and so on.