If the symphony is say for example in C major, likelihood is the selected clarinets for the piece may be in B flat, because, the clarinet’s part can be then written in D major, that is a relatively simple key with only 2 sharps. However, say the symphony is in A major, the selected clarinets would obviously be in A, because, the clarinet’s part would be then written in C major simple key no sharps or flats. Trumpets, horns, and a few other
orchestral instrument require transposition to boot. A poignant point: some 20th
century composers Prokofiev was infamous for doing this can write their full orchestral scores/compositions in what is called “live performance pitch. ” This means, all of the contraptions are written in the very same key. What occurs then?Music publicists would then take the composer’s full orchestral score which they ought to anyway.