Known in the US as alto horn, in Germany as althorn, and in the UK as tenor horn, this brass instrument pitched in Eb has a conical bore (gradually widening), and normally uses a deep, cornet-like mouthpiece. It is most commonly used in marching bands, brass bands and similar ensembles, whereas the French horn tends to take the corresponding parts in symphonic groupings and classical brass ensembles.

The tenor horn has a mellow sound, blends well, and serves as the voice between trombone (or baritone horn, in Bb below) and trumpet, cornet or flugelhorn (in Bb above). Its lowest note is concert A below C below middle C, and its highest is Eb above C above middle C. Its music is transposed a major sixth up from sounding pitch and notated in the treble clef.

It was invented as a middle voice in the group known as "saxhorns" in the mid-1800s by Adolphe Sax, a Belgian who is best remembered for the saxophone.

It has been made in various forms: most common is a sort of mini-tuba shape, with the bell pointing upward, which may help the voice blend before reaching the audience; the solo horn looks like (and indeed effectively is) an enlarged flugelhorn, with the bell pointing forward, projecting more toward the audience; another variant has the bell facing backward (for military marching bands that preceded the soldiers, thus helping them hear better and keep better time in marching). Of these types only the standard upright instrument is seen in UK brass bands.