I have noticed in advertisements that some bassoons are listed as Key of D Bassoons. From previous knowledge, I was unaware that bassoons were anything but key of C.

Is there special music for such bassoons, is the musician supposed to transpose existing music, or does this not mean what I think it does?

Thanks for any help you can render.

I think this refers to the high d key and high c key tha come on profesinal bassoons.