Don't misunderstand my above post - I am well aware that a trombone is normally not considered to be a transposing instrument, and that its lowest note is a Bb when reading from bass clef. As has already been said, when reading from treble clef it effectively becomes a transposing instrument.

However, there is such a thing as a Bb trombone, so it is wrong to say there isn't! That's all I was saying!