The Probert Encyclopaedia of Music

CLEF

In music, a clef is a sign placed at the beginning of a stave to indicate the pitch and name of notes. There are now only three clefs in use: the treble or G clef; the base or F clef; and the C clef. The G clef consists of the five highest lines, the F clef of the five lowest lines, of the great stave. The C clef may have three positions. It may be placed upon the first or lower line of the staff, in which case it is called soprano clef, upon the third line, in which case it called alto clef, or upon the fourth line, in which case tenor clef. It rarely or never is placed upon the second line, except in ancient music.
Research Clef