The American name for what is known in the UK as a tenor horn. A brass instrument pitched in Eb with a conical bore (gradually widening) and a deep, cornet-like mouthpiece which produces a mellow, rounded tone.

Most often used as a middle voice, supporting the melodies by the trumpets, cornets or flugelhorns, and fills in the gap above the lower tenor and bass instruments (the trombone, baritone horn, euphonium and tuba). Solos for the alto horn are very occasional, and are usually taken by the solo horns.

Most alto horns are pitched in Eb and are transposing instruments. Their typical range is from the A an octave and a minor third below middle C to the Eb an octave and a minor third above middle C (A2 to Eb5).

The standard bell-up horn comes in two basic shapes, one with the beginning of the bell looping over the top of the valves and the other looping below the valves.

It is most commonly used in marching bands, brass bands and similar ensembles, whereas the horn tends to take the corresponding parts in symphonic groupings and classical brass ensembles.