I think there are a number of factors behind the transposing instrument
phenomenon, including tradition.  Clarintetists, who routinely must switch
between their B-flat and A instruments in mid-piece appreciate having
essentially the same fingerings for each written note, regardless of which
instrument they're playing.  Incidentally, never let your child grow up to
be a clarinetist.  They are always being blamed for intonation problems in
the wind section since they often have to pick up a cold instrument in the
middle of a movement and play it without any opportunity to test or tune
it.

"Richard Todd" <[log in to unmask]>