C Clef Pitches Theory Things   William 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  
 

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.)