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Boss Trombone Rotary Is Different

For some reason, the markings and settings of the rotary in the bass trombone are usually exactly opposite from those in the French horn rotary. In general, if you just interchange the letters A and B in Fig. 28, the diagrams will fit the bass trombone rotary. The by-pass is open and the E and F slides are closed when the marks indicate a quarter after 12 o'clock, and the bypass is closed and the E and F slide tubing is open when the marks indicate 9 o'clock.