The baritone horn is a member of the brass instrument

Brass instrument

A brass instrument is a musical instrument whose sound is produced by sympathetic vibration of air in a tubular resonator in sympathy with the vibration of the player's lips...

 family. The baritone horn has a predominantly cylindrical bore as do the trumpet

Trumpet

The trumpet is the musical instrument with the highest register in the brass family. Trumpets are among the oldest musical instruments, dating back to at least 1500 BCE. They are played by blowing air through closed lips, producing a "buzzing" sound which starts a standing wave vibration in the air...

 and trombone

Trombone

The trombone is a musical instrument in the brass family. Like all brass instruments, sound is produced when the player’s vibrating lips cause the air column inside the instrument to vibrate...

. A baritone horn uses a large mouthpiece much like those of a trombone or euphonium

Euphonium

The euphonium is a conical-bore, tenor-voiced brass instrument. It derives its name from the Greek word euphonos, meaning "well-sounding" or "sweet-voiced"...

, although it is a bit smaller. Some baritone mouthpieces will sink into a euphonium's mouthpiece tube. It is pitched in B, one octave below the B trumpet. In the UK the baritone is frequently found in brass bands.