The trombone consists of a cylindrical tube bent into an elongated "S" shape (it is interesting to note that in French, trombone also means "paper clip"). Most trombones are slide trombones;. The section immediately following the mouthpiece is a short straight length of tube called the lead pipe. Below that is the slide, which allows the player to extend the length of the instrument, lowering the pitch. Some trombones have valves instead: see valve trombone, below.
Until around the 18th century, the trombone was called the sackbut in English. This was not a distinct instrument from the trombone, but rather a different name used for an earlier form (other countries used the same name throughout the instrument's history). The sackbut was slightly smaller than modern trombones, and had a bell that was more conical and less flared.
Trombones come in three sizes: alto, tenor, and bass. The soprano trombone does also exist, but is very rare and difficult to play.
The standard tenor trombone has a fundamental note of B flat (but is usually treated as nontransposing, see below). Since trombones have no valves or keys to change the pitch by a definite amount, trombonists memorize seven slide positions. The slide is said to be in "first position" when it is retracted all the way, and in "seventh position" when it is almost completely extended. Extending the slide from one position to the next lowers the pitch by one semitone; thus, for each note in the the harmonic series a downwards interval of up to a tritone may be added to the 1st position note, taking the lowest note of the standard instrument to E natural below the bass clef. Trombones often come with an extra piece of tubing attached, allowing the player to lower the pitch by a fourth by pulling a trigger, making faster passages and legato playing easier, and extending downwards the range of the basic tenor trombone. Playing with this trigger down modifies the set of positions; the distance between each is longer due to the lowered pitch. In fact, there are only six real positions available to the player, since the slide is too short for what is now really a trombone in F.
The bass trombone is also built in Bb and played in C. It is basically the same length as the tenor trombone but has a larger bore size, and has two valves, generally in F and D (although sometimes Eb), which change the key of the instrument, making it easier to play lower notes. This also allows the player to bridge the entire gap between the second harmonic and the fundamental. The notes on the bass trombone are played in the same position on the slide as the tenor trombone (until you start using the valves). There is usually one bass trombone player in a standard symphony orchestra, and they are also often seen in swing bands, wind ensembles, and a variety of brass groups. Wagner's Ring Cycle also calls for a contrabass trombone, pitched an octave lower than the bass trombone.
The alto trombone is pitched in E-flat, and is smaller than the tenor trombone. Because of its shorter length, the slide positions are different than on the tenor and bass trombones. The alto trombone is rarely used outside of the symphony orchestra.
There are also valve trombones, which have the same tonal range as the tenor trombone but a somewhat different attack, as they are set up like very large trumpets. Valve trombones are relatively rare. Some musicians consider them difficult to play in tune, although a small minority prefer them to the more common slide trombone. Other instruments with similar range and tone quality are the baritone horn and euphonium. Wagner also wrote a part for a bass trumpet in his Ring Cycle; this part is normally played by a trombonist. There are also a handful of other works in the classical repertoire which use this instrument.
The trombone (unlike most brass instruments) is not normally a transposing instrument and reads off of the bass clef (especially bass trombones), although it is not uncommon for trombone music to be written in tenor clef, or sometimes even alto clef. In brass band music, however, the trombone is treated as a transposing instrument in Bb and reads off the treble clef. By happy coincidence, this puts the notes in exactly the same stave position as they would be if the music were written in a (non-transposing) tenor clef, though obviously the key signature will be different.
As with all brass instruments, progressive tightening of the lips allows the player to jump to a different partial[?], up the harmonic series. In the lower range, significant movement of the slide is required, but for higher notes the player need only use four or less positions of the slide, since the partials are closer together. However, the higher notes may also be played in alternate positions; for example, F natural (at the bottom of the treble clef) may be played in both first, fourth and sixth positions.