The euphonium can use either the bass or the treble clef. When reading music in the bass clef, the music will be in concert pitch, so the note you see is the note you hear. For some reason, and I don't know why, the euphonium will sound a whole step lower than the notes written in the treble clef. But the fact that you can read music in both clefs means you can switch to the euphonium from most instruments.