Baritone
In this section
What is it?
A conical-bore B♭ brass instrument in the tenor–baritone range — and the tangle of names (baritone, tenor tuba, saxhorn) that surround it.
Why do Americans call it a baritone
American school-band tradition absorbed the euphonium under the looser label 'baritone,' partly from earlier smaller-bore instruments and partly from the manufacturing and pedagogy of US school bands. The instrument is usually a euphonium regardless of the name on the case.
Euphonium vs baritone — what's the difference
In British usage they are two different instruments: the euphonium has a wider conical bore and darker, fuller tone; the baritone horn is narrower and brighter. In American usage 'baritone' is often just a loose name for a euphonium.