The trombone family, like other instrument families, includes all the voices of the vocal choir.   Mostly we use the tenor or bass trombone, but other sizes are used on occasion.  The alto trombone, usually pitched in Eb, is having a renaissance, and can be found in many symphony and chamber orchestras and trombone choirs.  It is often used for playing music from the 16th to 18th centuries.  Alto trombone parts are often written in the alto clef, but as the presence of the alto clef does not necessarily mean an alto trombone is preferred, user discretion is advised.