The tenor-bass trombone is approximately the size of a tenortrombone, but for notes in the low register the bass trombone register it has an extra length of coiled tubing that can be engaged by pressing a valve.
Much orchestral music of the nineteenth and twentieth centuries also calls for bass trombone, but since the true bass trombone is a very long, very unwieldy instrument, most bass trombone parts are played today on a hybrid instrument called the tenor-bass trombone.