The Alto Clarinet is a larger clarinet in E-flat pitched a major sixth below the written notes. Traditionally, this has been the instrument used in the band. However, the Alto Clarinet has had a troubled life in the band world. As early as the 1950s, band directors were calling for its removal. The instruments were of poor make, composers did not write interesting or important parts for the instrument, and players did not take it seriously.
The E-flat Alto Clarinet is never used in the orchestra. Instead, when a clarinet of this pitch is needed, the Basset Horn (F Alto Clarinet) is used.
In fact, I only know of one piece in the entirety of the band literature where the Alto Clarinet is altogether indispensable, the rightly famous Lincolnshire Posy. In this piece, there are two prominent solos. In the first solo, in the 3rd movement, the Alto Clarinet forms a quartet with Piccolo, Oboe, and Bassoon, in one of the most unusual combinations in all of the band literature. The Alto Clarinet and the Piccolo play together in octaves, while the Oboe and Bassoon do likewise in canon. The second solo comes in the 6th movement where it is again paired with the Piccolo in octaves. Grainger, in letters to band directors, urged them to use as many Alto and Bass Clarinets as possible in this piece. And that, my friends, is the extent of creativity for the Alto Clarinet thus seen in the band.
The Alto Clarinet is almost always keyed to the low E-flat; though some older instruments may only descend to E (these instruments should be avoided as they are of poor make). Players upon the Alto Clarinet have remarked that the ease of production of the highest notes is the easiest of all the members of the clarinet family.
For a comparison between the F Alto and the E-flat Alto, read this article.
Grainger’s Lincolnshire Posy – 3rd Movement. Opening quartet is Piccolo and Alto Clarinet in canon with Oboe and Bassoon.