This clef points to the line (or space, rarely) representing middle C, or approximately 262 Hz
Positioned here, it makes the center line on the staff middle C, and is referred to as the "alto clef." This clef is used in modern notation for the viola. While all clefs can be placed anywhere on the staff to indicate various tessitura, the C clef is most often considered a "movable" clef: it is frequently seen pointing instead to the fourth line and called a "tenor clef". This clef is used very often in music written for bassoon, cello, and trombone; it replaces the bass clef when the number of ledger lines above the bass staff hinders easy reading.
C clefs were used in vocal music of the classical era and earlier; however, their usage in vocal music has been supplanted by the universal use of the treble and bass clefs. Modern editions of music from such periods generally transpose the original C-clef parts to either treble (female voices), octave treble (tenors), or bass clef (tenors and basses).