More formally that's an "F attachment", lowering the key of the instrument (assuming a standard tenor trombone) from B♭ to F. https://en.wikipedia.org/wiki/Trom

More formally that's an "F attachment", lowering the key of the instrument (assuming a standard tenor trombone) from B♭ to F. https://en.wikipedia.org/wiki/Trombone#Valve_attachments https://en.wikipedia.org/wiki/Trombone#Valve_attachments See also https://en.wikipedia.org/wiki/Brass_instrument#Trigger_or_throw https://en.wikipedia.org/wiki/Brass_instrument#Trigger_or_th...