From Wikipedia:

A clef (French: clef "key") is a musical symbol used to indicate the pitch of written notes.[1] Placed on one of the lines at the beginning of the staff, it indicates the name and pitch of the notes on that line. This line serves as a reference point by which the names of the notes on any other line or space of the staff may be determined. Only one clef that references a note in a space rather than on a line has ever been used.

I wanted it to, but $50 was pretty steep!