A monochord is an ancient musical and scientific laboratory instrument. The word "monochord" comes from the Greek and means literally "one string." In the monochord, a single string is stretched over a sound box. The string is fixed at both ends while a moveable bridge alters pitch.

The monochord can be used to illustrate the mathematical properties of musical pitch. For example, when a monochord's string is open it vibrates at a particular frequency and produces a pitch. When the length of the string is halved, and plucked, it produces a pitch an octave higher and the string vibrates at twice the frequency of the original (2:1). Half of this length will produce a pitch two octaves higher than the original—four times the frequency (4:1)—and so on. Monopipe is a wind instrument which serves the same purpose as the monochord.

In 1618, Robert Fludd devised a mundane monochord (also celestial or divine monochord) that linked the Ptolemaic universe to musical intervals. An image of the celestial monochord was used on the 1952 cover of Anthology of American Folk Music by Harry Everett Smith and in the 1977 book The Cosmographical Glass: Renaissance Diagrams of the Universe (p. 133) by S.K. Heninger, Jr., ISBN 9780873282086.

See also

• Đàn bầu
• Duxianqin
• Ektara
• Musical bow
• Berimbau
• Ichigenkin
• Diddley bow
• Long string instrument