A slide is a part of a wind instrument consisting of two (or more) pieces of tubing fitted one closely inside the other, and used to vary the overall length of the tube, and therefore the pitch of the instrument. Often two sets of tubes are used, with a U bend separating them; This arrangement is still called a single slide.
Slides are used in four main ways:
In instruments such as the trombone and slide whistle, moving the slide is the main way of selecting the note while playing.
In instruments such as the flute, the slide is a means of tuning the entire instrument, and is not normally moved while playing.
In instruments such as the trumpet, the slide is moved while playing to help correct the pitch of the note selected by other means.
In instruments such as the french horn, there are many slides used to tune the instrument, sometimes even during practice