Clef
Sign fixing the pitch reference.
What it means
The clef fixes which note lies on which line of the staff. Without it the staff shows only distances but no pitches — any note could mean anything. The clef supplies the one fixed reference point from which everything else follows. It is thus the first information that must be read at the beginning of a line, even before the key signature.
In the notation
Three kinds are in use, each named after the note it fixes. The G clef encloses with its spiral the line of the once-marked g and today stands on the second line — that is the treble clef. The C clef marks with the meeting point of its curves the once-marked c and has remained movable: on the third line it gives the alto clef, on the fourth the tenor clef, on the first the soprano clef. A small figure 8 below or above the sign shifts the reading by an octave. At the beginning of every system the clef is repeated; a change in the course of the piece is indicated by the new sign at the point concerned, usually in smaller form.
Musical use
The point of this variety lies in avoiding ledger lines: every voice is to get the clef in which its range fits as comfortably as possible onto the staff. In music before the nineteenth century, far more extensive use was made of this than today; a four-voice vocal score could show four different clefs. Today the selection is limited, but viola, cello, bassoon, and trombone continue to change clef depending on register.
Other names
In professional usage one usually speaks simply of the “clef.” What is always meant is the sign at the beginning of the staff that assigns a particular pitch to a line; the three basic forms are the G, F, and C clefs.
Performance practice
Look at the clef at every change of line before you come in — with parts that change clef, this is precisely the most common source of error. Learn a new clef through fixed anchor notes, not by calculating from a familiar one. Mark change points in your part before you play through them for the first time.