C Clef Pitches Theory ThingsWilliam Wieland
 Alto Clef — the middle line is middle C (C clefs "point" at middle C.)

CDE FGA BCD EFG ABC
 
 
 Tenor Clef — the second line from the top is middle C

ABC DEF GAB CDE FGA
 

C clef mnemonic device

Violas usually use the alto clef. It is sometimes called the viola clef.

Bassoons, Cellos, and Trombones occasionally use the tenor clef.

To practice reading C clef pitches on your computer, click Note Identification set to alto and tenor clefs.   (musictheory.net)

Alto clef reading: Some cantus firmi     A Gloria and Victimae pascháli láudes

Tenor clef reading: Lassus Bicinium 23 (Scroll for the second page.)
Note: is the same letter in any clef.