[Dixielandjazz] Bb is the ...

[Marty Nichols]

No, no, it's not that Bb is the *key* we should play in, it's a tuning note.
 
All the trombone players of the world use it, and arranged for nature to play it so we can tune our horns.  If we use Bb as our tuning note, we can adjust our tuning slide to compensate for the restrictions of the "closed slide position".  Ask any trombone player, to explain if you don't get it.
 
Gerry Marty Nichols
Rockledge, FL