A trombone is a wind instrument belonging to the brass family. It has a cup-shaped mouthpiece and consists of two long U-shaped cylindrical tubes. One of the tubes, known as the crook, is a sliding mechanism. The other tube expands at one end into a bell shape. Sound is produced when air is blown through the lips, which are stretched across the mouthpiece. The pitch of the instrument is altered by moving the slide back and forth. Trombones produce rich, sonorous tones and are made in different sizes, the tenor and bass instrument the most used. The basic design of the instrument has changed little since it was introduced in the 15th century. It was used for ceremonial and church music until the 18th century, when it appeared in operatic orchestras. Trombones are used in symphony orchestras, symphonic and brass bands, and jazz ensembles.
The trombone was developed from the medieval sackbut, made in five sizes: soprano, alto, tenor (and tenor-bass), bass and contrabass of which only the tenor-bass is normally used today, the parts written for the alto and the bass being played on the one instrument.
The very few parts written for the soprano are played on the trumpet and those for the contrabass, such as in Wagner's Ring, are usually played on a tenor-bass with a second set of auxiliary tubing. The trombone's most characteristic feature is the slide, by means of which the tube can be adjusted to different lengths in seven positions, so that all the notes of the chromatic scale can be produced as natural harmonics. The trombone was thus a chromatic instrument long before the horn and trumpet became so by the invention of the valves. The intonation, as in string instruments, is not fixed, but depends entirely on the player's ear and skill. Many notes are, of course, available in more than one position (as different harmonics), so that the player often has the choice between an easier and a more difficult way of passing from note to note. A strict legato between notes in different positions is not possible, as the breath has to be interrupted during the change of the slide; the uninterrupted movement from one position to another is used only when the part is marked glissando (sliding).
The range of the tenor trombone (in B flat) is from E below the stave in the bass clef to D on the fourth line of the treble stave and three pedal notes (B flat, A and A flat) below the bass E. On the tenor-bass trombone the gap between the low E and the pedal B flat is filled by means of extra tubing, which when brought into action by a switch turns the instrument into a trombone in F. The alto trombone (in E flat) has roughly the same range as the tenor but a fourth higher, the bass trombone (in F) the same a fourth lower (the bass trombone in G is virtually obsolete), and the contrabass trombone (in B flat) an octave lower, but the pedal notes on these three instruments are seldom asked for. In the 19th century valve trombones were invented and gained favor in military and brass bands as being easier to play, though even there it is admitted that their tone is inferior to that of the slide trombone. The valve trombone never gained a firm footing in the orchestra in Western Europe.