From jazz in the 1950's. Recording sessions featuring musicians assembled for the project - i.e. not a regular working/gigging group. You might get ample rehearsal time or not, depending on the size (and interest) of the record label; either way, it meant the emphasis would be on blowing, and not on things like composition or interaction. It gave sidemen like John Coltrane a chance to lead their own groups, if only for a day or two at a time.