Bass clef or treble clef
Euphonium parts come in two forms. In bass clef they're written at concert pitch, like the trombone. In treble clef they're transposed in B♭, a major ninth above the sounding pitch, just like the tenor sax. Brass bands use treble clef.
What it's for
Many euphonium players started on trumpet or cornet and read treble clef. Switch a bass-clef part to treble clef in seconds: for a concert C, the player reads a D a ninth higher.
Frequently asked questions
What if my euphonium player reads bass clef?
No transposition needed: give them the concert-pitch part.
Does it work for baritone horn?
Yes, treble-clef baritone transposes the same way.