Music for the bass and contrabass tubas is generally non-transposing and written in bass clef. The instrument is pitched in the contrabass register.
In wind band scores notation for the contrabass tuba in Bb is subject to national variations.
In Germany, Italy and England the notation is non-transposing in bass clef.
In France, Belgium and the Netherlands it is transposing, with no accidentals, in bass clef (sounds a major second lower than written):
In Switzerland transposing, in treble clef, no accidentals (sounds two octaves and a major second lower than written):