notochord

n. [ L. monochordon, Gr. &unr_;, fr. &unr_; with but one string; &unr_; only, single + &unr_; 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. [ 1913 Webster ]