How many forms of clefs are there in modern music notation?

A clef (from French clef "key") is a musical symbol used to indicate the pitch of written notes. Placed on a stave, it indicates the name and pitch of the notes on one of the lines. This line serves as a reference point by which the names of the notes on any other line or space of the stave may be determined.

There are three forms of clef used in modern music notation: F, C, and G. Each form assigns its reference note to a line (and in rare cases, a space) depending on its placement on the stave.

The use of different clefs makes it possible to write music for all instruments and voices, regardless of differences in tessitura. Because the modern stave has only five lines, it is not possible to represent all pitches playable by the orchestra with only one clef, even with the use of ledger lines. The use of different clefs for various instruments and voices allows each part to be written comfortably on the stave with a minimum of ledger lines. To this end, the G-clef is used for high parts, the C-clef for middle parts, and the F-clef for low parts—with the notable exception of transposing parts, which are written at a pitch different from their sound, often even in a different octave.