Illustrated Companion to the Latin Dictionary/Tetrachordon

This article incorporates text from a publication now in the public domain: Rich, Anthony (1849). The illustrated companion to the Latin dictionary, and Greek lexicon. p. vi. OCLC 894670115. https://archive.org/details/illustratedcompa00rich. 

TETRACHOR'DON (τετράχορδον). Literally, having four strings, and thus producing four notes (Capell. ix. 324.); a scale comprising two tones and a half, which formed the old Greek musical system. The water organ (hydraulus) is termed a tetrachord by Vitruvius (x. 8. 2.), when it has only four barrels.