Octave clef
A clef with a small 8, sounding an octave away.
What it means
An octave clef is an ordinary clef with a small 8 attached, which indicates that the notated text sounds an octave higher or lower than written. If the 8 stands below the sign, everything sounds an octave lower; if it stands above, an octave higher. The purpose is practical: it saves ledger lines and allows a part to be read in familiar notation even though it lies outside the usual range of that clef.
In the notation
Most often the treble clef with a lowered 8 is encountered. In it the tenor voice in choir and the guitar are notated — both sound an octave below what is written. The bass clef with a lowered 8 is found with the double bass, whose part likewise sounds an octave lower. The 8 above the treble clef is rare. This permanent marking is to be distinguished from the octave sign above or below a single passage, which applies only for its duration and is cancelled again with a dashed line and the note loco.
Musical use
In choral scores the notation of the tenor voice is not uniform: it appears in the treble clef with an 8, in the treble clef without an 8 — then the octave shift is meant silently — or in the bass clef sounding as written. Whoever creates or compares choral parts should know these three possibilities. With the guitar, the octave shift is so self-evident that the 8 is often omitted; the instrument nonetheless sounds an octave lower, which needs to be borne in mind when playing together with keyboard instruments.
Most common is the treble clef with a small 8 below it, the tenor clef for guitar and tenor voice.
Performance practice
Check with every new part whether the clef carries an octave indication, and when checking against other instruments always reckon in sounding pitches. When communicating pitches within an ensemble, say explicitly whether you mean written or sounding pitch. When studying with recordings it helps to become consciously aware of the octave register once — otherwise the ear silently corrects, and the confusion is noticed only late when playing together.