The clef is a sign at the beginning of the staff that indicates pitch. It identifies the position on the staff of a single pitch for which the clef is named. We use one clef or another because we are anchored in a certain range of notes (or a certain part of the piano keyboard, if you will). In early music, up to ten different clefs were used in order to keep most of the notes on the staff. Ledger lines lessen the the need for so many clefs. If you are familiar with piano music, then you know that the same middle C on the piano is just below the staff in the treble clef of the right hand and just above the staff in the bass clef of the left hand (in both case, on the first ledger line).

When a composer or arranger or editor selects a clef, he or she does so to keep most notes on or near the five lines of the staff. This makes music easier to read than it would be with many outliers on many ledger lines—assuming, of course, that the reader has trained using that clef. That is the trade-off: the power of a tool versus the learning curve required to use it.

Most clefs are named after vocal parts (soprano, tenor, bass, and so on). Some orchestral instruments use “alternate” clefs; e.g., the viola part is written in alto clef. You will find more clefs in older editions of older music. In modern printings, there is some editorial judgment involved. In my experience, French publishers use more clefs than American publishers, probably reflecting their own training traditions.

The clefs have a related use within the orchestra: Sometimes an instrument has to transpose. The musician is looking at music in one key but has to play it in another key. The simplest solution is to change instruments when that is possible (replace a B-flat clarinet with an A clarinet, for example). More difficult is to translate each note sequentially. A middle ground (again, if you have drilled with clefs) is to imagine that the music is written in another clef. Changing the clef “moves” the notes; a note on the first ledger line of one clef becomes some other note depending on the which clef you pretend it is written in. This strategy, by the way, is not purely mechanistic, since you still have to think about the key and the sharps and flats involved. But it can be a handy tool. The clefs also appear in the conductor’s score when needed to clarify an instrument’s part.