The tenor horn in British English, Althorn in Germany, Alto Horn in US;
It has a predominately conical bore (most tube extents 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 horn tends to take the corresponding parts in symphonic groupings and classical brass ensembles.
The alto horn is a valved brass instrument (in E flat) which has a predominately conical bore like the euphonium and flugelhorn.
It uses a deep funnel- or cup-shaped mouthpiece.
The alto horn's conical bore and deep mouthpiece produce a mellow, rounded tone which is often used as a middle voice, supporting the melodies of the trumpets, cornets or flugelhorns, and filling the gap above the lower tenor and bass instruments (the trombone, baritone horn, euphonium and tuba).
Its valves are typically, though not exclusively, piston valves.