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The trombones—along with trumpets, horns, and tubas—make up the main groups of the brass instrument family. The name “trombone” means large trumpet, and that is how they were conceived.
The individual trombones that make up the group include the slide trombones — soprano trombone, the alto, tenor, bass, and contrabass trombones — and the valve trombone, which features valves in place of a slide. Of these, the alto, tenor, and bass trombone are used in the contemporary orchestra, with the tenor being the most often used in jazz.
Some slide trombones feature a valves or trigger that lowers the pitch and add to the range. This is a different situation than the valve trombone, which has no slide. The tenor trombone may have an F attachment, in which case, it may be referred to as a tenor-bass trombone, while the bass may have both an F and an E or D valve.
Trombones have either six or seven playing positions, specific placements of the slide, at each of which several pitches are available. In sixth or seventh position, depending on the instrument, the slide is completely out. In addition, a number of pitches can be created at multiple positions, while some can be played only at one position. A passage in which a player moves mainly between adjacent positions will be easier to play than one in which the player has to move between extreme positions.
From: http://www.wisegeek.com/what-is-a-trombone.htm