Professional alto trombone. The 36H offers performance advantages in a number of ways:
Its sound is highly characteristic of a true alto trombone (and not say, of a small tenor trombone or "slide flugelhorn").
Its bell rim and slide positions are in the same relative location as those on a tenor trombone, making transition to the alto that much more seamless. This can be a real advantage if you are an alto-tenor doubler.
The Bb attachment functions in the manner as an F attachment on a tenor trombone, giving F (4th line bass clef) in 1st and E natural (third space) in flat second instead of 6th and 7th (similar to C and B on tenor); allows the notes from Ab first line down to E to be performed (these are missing on a straight alto, in the same way that "trigger" Eb down to B are missing on tenor); and gives numerous trill possibilities in the upper register. -cdp