High quality large shank trombone mouthpiece by Bach.  Large shank mouthpieces fit in what are sometimes called symphony tenors - horns with a .547" bore like the Bach 42.  These are common in the band world, and the most common size among orchestral players.  Some jazzers use a big horn too but not many.  Bass bones, .560" bore like a Bach 50, accept large shank mouthpieces as well.  If your trombone takes a large shank piece, this one's for you.