Alto clef is used for printing music for instruments that are played below the staff lines of treble clef. When music is printed in alto clef, the instrumentalist does not have to read using ledger lines, the short lines used for notes below the staff. The third line in the alto clef staff represents middle C.
The primary instrument that uses alto clef in modern orchestras is the viola. The viola is very similar to the violin, except it is a little bit larger and therefore lower and richer in tone. It appears in string quartets, which include two violins, a viola and a cello, and in classical orchestras. Alto clef is sometimes called viola clef because the viola is the only instrument whose music is always printed in alto clef.
The English horn is an F instrument, meaning that the note that looks like a C on English horn music sounds like an F when played. Publishers do this to avoid having to print whole pieces on ledger lines. In some classical music, particularly in Russian compositions, English horn parts are written in alto clef instead of being notated a perfect fifth above pitch. English horn players must be able to read both treble and alto clef in order to read all of their music.
Trombone players may be expected to read treble, alto, tenor and bass clefs at various points in their careers. Lead trombone parts that carry melodies, called first trombone parts in orchestras and bands, are often written in alto clef, so trombone players who hold prominent chairs in their ensembles must be able to read it. Meanwhile, the second trombone part may be written in tenor clef and the third trombone part in bass clef.
The bassoon has a wide range, most of which falls into the bass and tenor clefs. Bassoons are used mostly for the rich, dark tones at the bottom of their range, so bassoon players spend most of their time reading bass clef. The instrument's middle range, written in tenor clef, is powerful but somewhat pinched in tone. Bassoonists only need to read alto clef when the piece they play primarily uses the top of the instrument's range, which produces the bassoon's weakest and thinnest tone.