Mon"o*chord (?), n. [L. monochordon, Gr. &?;, fr. &?; with but one string; &?; only, single + &?; string: cf. F. monocorde. See Chord, and cf. Mainchord.] (Mus.) An instrument for experimenting upon the mathematical relations of musical sounds. It consists of a single string stretched between two bridges, one or both of which are movable, and which stand upon a graduated rule for the purpose of readily changing and measuring the length of the part of the string between them.