Physical instruments aren't really tied to a key. Transposing instruments only even exist because sight-transposing is so difficult. There is nothing fundament

Physical instruments aren't really tied to a key. Transposing instruments only even exist because sight-transposing is so difficult. There is nothing fundamental about an instrument that makes it transposing. We only use that kludge because of the difficulty sight-transposing. Suppose you do have a transposing instrument though. It's in Bb, and you'd like to play music written for an instrument in F. Very few musicians can do that without hesitation. The same applies when there is a desire to shift the entire piece of music up or down. Perhaps you are using a hecklephone instead of a lupophone, so you shift everything up. Perhaps you just lost a trumpet player with an unusually high range, and thus need to shift everything down. It would be great to be able to expect all players of ordinary skill to be able to do this without hesitation. BTW, I'm aware that one needs to avoid troublesome notes and things like impossible trombone glissandos. This doesn't change the general truth of the above.