This is an often pondered question for Beginner Trombonists but an important one to get right from the outset.

Many teachers will ask (or should ask) their trombone students whether they want to learn the treble or bass clef.

This can be decided by the teacher if the student does not know, but should really be decided depending on the type of ensemble (group) that the learner will hope to progress to when they become experienced enough.

Often this causes great amounts of confusion to already daunted students and parents, so I will simplify.  In a brass band, Trombones (with the exception of the Bass Trombone) are required to be able to read the treble clef.  In wind bands, orchestras and most jazz groups the trombone reads the bass clef.  It is a simple matter of finding out what the student's interests are, where they want to progress, and of couse whether or not they read either of these clefs already, from playing another instrument for instance.

All tutor books are available in both Treble and Bass Clef so it should be a simple choice made by students and teacher that all understand.