The Etiquette of Improvisation
Thus, for musicians of my era, improvising meant picking a tune, from among the ones we all knew, as the basis for the collective work. [...] The agreement to keep some things fixed and vary others made it possible for a group to sound like it knew, collectively, what it was doing: to not get lost, to have some idea of what might be coming next, to interpret what the others did as hints of a direction the collective effort might take.
This is from an essay I’m reading, by Howard S. Becker, titled The Etiquette of Improvisation (via Håkan). I’ll have to add etiquette to my list over things that guide and constrain human expression/creativity. See also my posts “Jazz and Semiotics,” and “The Importance of Archetypes in Improvisation.” And Becker has several articles on his homepage that look interesting.