The tenor trombone is the standard beginner and general-purpose instrument, with a lighter build, standard bore sizing, and the largest catalog of beginner method books and school band parts written for it. The bass trombone has a larger bore and bell for a deeper, more powerful low register, and typically includes one or two additional rotary valves to fill in low notes the slide alone can't reach efficiently, adding both weight and mechanical complexity. Because bass trombone parts are less common in beginner repertoire and the instrument itself is heavier and more expensive, it's almost always a second instrument chosen once a player has decided they specifically want to play low brass parts in bands or orchestras.