The Alto Trombone is a small trombone pitched traditionally a fourth higher than the Tenor Trombone. From the earliest days of the trombone entering the orchestra, the Alto Trombone was a key member to the standard trio of trombones (one Alto, Tenor, and Bass), but by the later parts of the Nineteenth Century, the Alto was slowly losing ground and it completely disappeared from the orchestra by the turn of the century. Only rarely afterward does it make an appearance in the orchestra as a special instrument. All this said, the Alto Trombone still exists and is becoming more widely used again by principal trombone players. Parts originally written for the Alto in mind are today being played on that instrument. This is a shift over years past when players only used the Tenor Trombone. In the Classical Era, the Alto Trombone was sometimes used as a solo instrument far more than the Tenor (a concerto exists by Leopold Mozart). As the Alto began to disappear from the orchestra, composers would often write solos for the second trombonist to ensure that the solo would always be played on the Tenor.
With hardly any exception, the Alto Trombone is only written in the alto clef, making it the only wind instrument to do so. Like all trombones it does not transpose, but is always written at concert pitch.
The traditional range of the Alto will go up to a written E-flat an octave and a minor third above middle C, though F’s above this are not unheard of. At the bottom of the range, it is fairly weak, and can go no lower than the A at the bottom of the bass clef. Remember that when writing for the Alto, more often than not, the instrument will not have an additional valve (akin to the F-attachment of the Tenor Trombone). However, a few professional instruments might possess a valve, most will not.
Outside of literature written during the Nineteenth Century, I know of no band work that utilizes the Alto Trombone as an independent voice. It is possible that part of the reluctance of using the Alto Trombone is that the trombonist will have to learn new slide positions when learning the instrument, but for professionals who use the instrument on a regular basis; this should not be a problem The sound of the Alto is much sweeter and softer than that of the Tenor. It does not have the same power or projection. It works very well as the top voice of a chorale.
As the slide is shorter, technique of the Alto will be quicker than on the Tenor. In its position at the top of the trombone choir it can effectively bridge the gap between the trumpets and trombones. It can also blend quite seamlessly with the Horns. With its lighter voice, it is more suited for combinations with the various woodwinds than other trombones. The only regularly available mute for the Alto Trombone will be the standard straight mute. It should be fairly easy to include an Alto Trombone in works written for college and professional level groups, and I see no reason not to include it in works written at this level. Performers will be glad to have something to play on their instruments other than Beethoven and Schumann.
The famous Tenor Trombone solo from Mahler’s 3rd Symohony (probably player here as a joke).
Leopold Mozart’s Concerto for Alto Trombone.
Schumann’s 3rd Symphony “Rhenish” 4th movement