Alto clef
The C clef on the third staff line.
What it means
The alto clef is one of the forms of the C clef. It fixes that the third staff line — the middle one of the staff — denotes middle C. From this one fixed note all the other pitches of the staff follow. The name comes from vocal writing: it was the clef in which the alto voice was notated. Today it is scarcely used there any more, but it has held on as the standard clef of the viola and is met with besides in scores of older music and in scholarly editions.
In the notation
The sign consists of two arcs opening to the right that meet at a vertical double line; the point at which the two arcs come together marks the line of middle C. With the alto clef this point lies on the middle line. Middle C, which in the treble clef needs a ledger line below the staff and in the bass clef one above, thus sits here in the middle of the staff. This is precisely its practical purpose: the range between bass and treble register can be shown without ledger lines. If the same sign moves to the fourth line, the tenor clef arises, on the first the soprano clef.
Musical use
The viola notates fundamentally in the alto clef and changes only in high positions into the treble clef. Trombone and bassoon too occasionally resort to it when their part runs for a longer time in the middle register. In music before about 1750 the choice of clef was in any case more flexible than today: every voice received the clef that mapped its range most economically, and the alto clef belonged to these as a matter of course. Whoever uses urtext editions or facsimiles therefore meets it regularly.
Performance practice
Do not learn the alto clef by converting from the treble clef — that works for the first weeks and blocks you afterward. Anchor instead two or three fixed reference points in the staff, for instance the C′ on the middle line and the A on the second line from below, and read everything else as a distance from them. It helps to write short familiar melodies in the alto clef and to sing them from sight; thereby the notation couples directly to the sound instead of to a rule of calculation. With parts that change clef frequently it is worth marking the change points in the notation before the first play-through.