clef

In music, a sign placed at the beginning of a stave to indicate the pitch of the written notes. It was introduced as a visual aid in plainchant notation, and takes the form of a stylized letter centred on a particular line to show the pitch of that line.

The standard clefs for most instruments are G (treble) and F (bass) clefs. The C clef is now comparatively rare, except for viola, high parts of the cello range, bassoon, and double bass.