I've been told several times that brass instruments prefer to play in keys with flats, rather than sharps. As a trumpet player myself, I cannot relate to this preference. E major is just as easy or hard as A flat major. I interpreted it as being the playing key, not the concert key.I'll take that the preference is true, since I've heard it from several trustworthy sources, but why is that? I have several hypotheses:1. Tradition. Bands (brass bands especially?) usually play in keys with flats, therefore that is what the players are used to, and more comfortable with. But how did the tradition start?2. Intonation. The construction of valve instruments make the intonation better in keys with flats. For example in A major: the low C# of the trumpet is naturally high, whereas a good intonation should have it low as the third of a major chord. I cannot think of a counterexample right now.I welcome guess answers, but please mention that you are just guessing if you do not actually know for a fact.
The reason most brass instruments are in the flat keys are because back then composers need to transpose because it turned out that playing a concert "C" on that instrument turned out to be a "D" on a brass instrument and so with that it was conceived the concept of instead of making an instrument tune down or to change the manufacturing so everything is in the key of C was first off too expensive at the time and would require people to relearn figuring. It boils down to players found it easier to not learn new fingering but composers had to remember how to write for those instruments because of the lack of change.