I play horn in my little town orchestra and I know how to transpose horn music. No problem, but I don't have the book for trumpet. (I've got the horn one.) I've been told to play the third trumpet part on the piece that does not have a horn part. I've got a b flat trumpet, the part is for trumpet in D. Is it up or down and what interval? The key is C, so how many sharps or flats? (See, horn in F plays an F for concert b flat. Trumpet is a b flat instrument, but plays C for b flat. It's different and changes things!) Rehearsal is at 7:30pm. Thanks!