The trombone is a brass instrument with a fairly low pitch. It has a cylindrical bore with a flared bell at the far end. It has a long, U-shaped tube between the mouth-piece and the main tube, which can be slid in and out, to lengthen or shorten the column of air, thus lowering or raising the pitch of the note produced.  The sound is produced by vibrations of the player's lips into the cup-shaped mouthpiece. When the lips are fairly slack, the low, fundamental tone is sounded. As the lips are successively tightened, and the air pressure is increased, the overtones of the harmonic series are generated successively. 

The slide is used to alter the pitch by semi-tone intervals, thus filling in the gaps between the notes of the harmonic series. This is equivalent to the valves on a trumpet or French horn, permitting the full chromatic scale to be played. However, it takes some time to move the slide from one position to another, so rapid passages cannot be played on this instrument, unlike the trumpet. In jazz the slide is often used whilst continuously sounding the instrument, generating a sliding pitch, or a glissando effect.

The trombone was used in early music especially in operas. It then seems to have been infrequently employed in orchestras until Beethoven's time and onward. It is now a standard instrument of the modern symphony orchestra, and also in brass bands, dance bands and jazz ensembles.

There are two types of trombone. The bass trombone has a range from B1 (just below the C 2 octaves below middle C) up to the F above middle C. The tenor goes from E2 (1 octaves below middle C) up to Bb4 (Bb above middle C).