[MusicTlk] Writing for a Bb Trumpet

[Mike Jolls]

Hello all

I'm a pianist and I'd like to write some music so that my nephew, who plays a Bb trumpet, can play along with me.  How should I write his music?  If I'm understanding his instrument correctly, when he sees and plays a C, he'll actually blow a Bb.  If that's correct, then he's a full step below me.  That suggests I should write his music in the key of D major so that he'll match me.

Am I correct, or is there something else I need to understand?

Thanks for the explanations.

Mike Jolls