Bass trombone music is written at concert pitch in bass clef. This was historically due to this part being taken by a G bass trombone, rather than the modern B♭ bass trombone. As instrument technology modernized, the need for a bass trombone in G diminished, with the introduction of the B♭ bass trombone with F rotary valve. The larger bore and open wrap of the F valve gave the B♭ bass trombone a lower available playing register than the straight G bass trombone.