Yours, Etc.: Origins and Uses of 8 Sign-Offs
1570s in a musical sense, "character on a staff to indicate its name and pitch," from Middle French clef (12c.) "key, musical clef, trigger," from a figurative or transferred use of classical Latin clavis, which had only the literally sense "key" (see slot (n.2)). In the Middle Ages, the Latin word was used in the Guidonian system for "the lowest note of a scale," which is its basis (see keynote). The most common is the treble, violin, or G-clef, which crosses on the second line of the staff, denoting that as the G above middle C on the piano.