Well, if the piece getting played is a *song*, then one would expect the singer to have a good deal of input, as one would consider it his-her tune. That said, unless the jam is held at someone's house (and the homeowner plays the role of the jamnazi), someone laying down rigid rules isn't very cool at all.

Here I'd say that the Dave Matthews Band or Phish have given you an odd concept of a "jam session." If you're at a bluegrbutt jam, expect the songs-instrumentals to roughly conform to the general aspects of bluegrbutt (i.e. generally three chords; generally in 2-4 time; most verses either 16 or 32 bars, with choruses of generally equal length; you don't play your breaks in polyrhythms or take Charlie Parkeresque high-energy bebop flights of fancy), just like you wouldn't break out your Lynyrd Skynrd licks if you sat in at a Dixieland jam. The idea of a jam session "unfettered by subjective preconception of what must occur" will generally not exist anywhere. A room full of 'grbutters will generally expect the music to sound like bluegrbutt, a Dixieland jam will generally conform to the standards and conventions of Dixieland, a blues jam will generally conform to the standards and conventions of the blues, et al, et c.

-- Lane Gray And they were both naked, the man and his wife, and were not ashamed. Gen 2:25 remove the .lead from my address