The small movable metal tube(s) of a brass instrument which lengthens or shortens the tubing of the instrument, thus changing the pitch to bring the instrument, in tune with other instruments in an ensemble. Typically, there is a slide associated with each valve, so they can be tuned individually. Pulling a slide out (or making the tubing longer) will lower the pitch or make the pitchflatter. Pushing a slide in (or making the tubing shorter) will raise the pitch or make the pitchsharper.