An Instrument in the Key of B♭, Yet the Music is Written in C

Usually, sheet music for brass instruments is written in the same key as the instrument itself. For the euphonium and tuba, however, music is written in the key of C, despite the instruments being in the key of B♭. This is based on orchestral conventions.
In the mid-19th century, when the tuba and euphonium appeared and started to gain acceptance in orchestras, there were not yet any dedicated tuba or euphonium players. Contrabass or trombone players would often switch and play the new instruments. As a result, historians surmise that composers favored musical notation that was easy to read, regardless of the type of instrument, or its key. So they chose to write in the key of C.
Conversely, brass band music teachers in the U.K., France, and elsewhere chose to instruct (young) beginners in how to read music and learn fingering. Students then gradually applied that same fingering to low-pitched instruments. That is why we currently may see sheet music written in the treble clef (for easy fingering) but intended for a B♭ instrument.