Bass Trombone
The bass trombone is normally the third or fourth trombone in an ensemble, and the instrument is designed to cover low notes not possible on a tenor trombone. Bass trombone players can play in a wide range, but much of the emphasis is in the low tones. With a single rotor trombone, most bass notes are possible.
The basic instrument has an F-attachment, which allows the trombone to be pitched in F as well as Bb. With an F-attachment, it can be either in 6th position or in 1st position with the F-attachment engaged (the trigger pulled). Below this note requires an F-attachment. The notes E, Eb, D, and Db (below the staff in the bass clef) can be played. Db is played by pulling the trigger and playing near 7th position -- as much tubing as a single rotor trombone has. The notes C and B natural are not really possible without doing some tricks (like retuning the F-attachment, etc.).
Hence the need for a double rotor bass trombone. The second rotor can be tuned (usually in "G") to allow for a true low C and B natural (two leger lines below the bass clef) to be played. It also allows for many other overtones and alternate positions to be used. Generally speaking, with a single valve (or rotor) almost every note can be played. A student learning bass trombone can play a large bore horn with a single valve. Rarely is a C or B natural called for. The C can be played with some work and the B natural is very rarely called for in most works.
Bass trombones for advanced players will always have double rotors, although you still see bass trombones for sale with a single rotor.
