Assertion: trombone's f attachment | /r/MadeOf | rotary valve
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text: A trombone F attachment consists of a rotary valve (similar to what is found on American made French horns) and an additional length of tubing in the bell section of the instrument so that when the valve is activated or rotated, air passes through the additional length of tubing to change the Bb trombone down a perfect fourth to the key of F.