How do these “Blues Jams” work, anyway?

I’m not sure how other jams do it, but typically the singer will call key, feel, and tempo. e.g. “This is a shuffle in E”. A lot of times people will describe what they want in terms of other songs or artists, “OK – Jimmy Reed feel in Bb”. The other common variation is the “quick 4” which is 12-bar except you go to the IV for the 2nd measure. I find that as the bass player, it often falls to me to make sure everyone (including myself) knows the key and feel. I think the default assumption at any blues jam is that you’re dealing with the 12-bar form. Any variation will (should) be specified before the song. Common exceptions to twelve bar that are frequently played are “Stormy Monday”, “Born Under a Bad Sign”, “My Babe”, “Key to the Highway” (8-bar), “The Thrill is Gone”, “Seventh Son”, “Wang-dang-doodle” Might be different at other Jams, but that’s what I see.