I am trying to arrange a song for our Church Praise Band (My first attempt at arrangement...the results should be interesting!). I understand that the accidentals should match the key signature. For instance, if writing in the key of Bb, the accidentals should be written as flats, not sharps. However, the song will be concert Bb, which of course puts the trumpet part in C.
When writing the trumpet part in this particular key, should the accidentals be written as sharps or flats? For example, should it be D# or Eb?