Bluegrass musicians are certainly excellent players, as a general rule, but I would take issue with the argument that they are at all comparable to jazz players when it comes to improvisation.

Bluegrass songs, like blues or even trad-jazz numbers and most folk and popular music, have fixed harmonic structures (chord progressions). They might not all be as simple or predictable as the 12-bar blues, but a reasonably competent player should be able to play along, if not immediately, certainly after listening to one or two "go-rounds."

This is not to say that a lot of inventive playing can't go on within the structure of these tunes -- just as seriously creative playing can also occur within the even-simpler structure of the blues. But it's not like modern "free-bag" jazz, where *everything*, even the chordal structure, is improvised.

I can play with bluegrassers (not saying how well I can do it, but I can surely strum along pretty much error-free), but I *know* I can't even begin to play with real jazz players.