A clef marking that may sit anywhere on the five line staff, and whichever line its center points to is middle C ( c1). This is also called the movable clef because it can signify a number of different clefs. The most common c clefs are the tenor and alto clef, where middle C is on the fourth line and the third line, respectively. Two rarely used c clefs are the mezzo-soprano clef and soprano clef, where middle C is on the second line and the first line, respectively.(Alto Clef and Tenor Clef) 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. Both the alto and tenor clefs are used in traditional choral scores.