WYSIWYG has basically got the right information, as I see it, with a few additional notes.

As I mentioned in other threads, we've had a slow jam at O'Hurleys General Store, in Shepherdstown, WV for some 22 years now, on Thursday nights. Several years ago, we saw the need for another jam at a train station in Martinsburg, which transmuted into a jam at an Irish bar in Fredrick, Md. on Tues nights.

Each jam has it's own unwritten rules, which vary in style and purpose, but which have almost the same effects.

The jams have a musical style. One has a mixture of Old Time and Irish on a single night, the other alternates styles (OT/Irish every other week.

The revolve around a given series of music books. One uses The Fidders Fake Book, the Waltz Books, and Rough Water collection, while the other uses a series of "100 Irish Session Tunes, etc. as well as Fiddlers Fake. Although neither has a great deal of singing, they both use Sing Out for follow along.

Each group has new songs/music introduced, but you must bring legible copies for everyone, and for several weeks there after, if you really want to have the song become part of the group memory.

Both involve musicians sitting a a circle, and the pick is passed around, with the person making the choice having three choices. He can call out a tune or a series of tunes (ala contra dance) and the number of repeats, he can sing a song himself, or he can ask another to sing a song or lead a tune (excellent for beginners to practice on a tune they want to learn).

THERE ARE RULES that must be obeyed, or the group WILL dissolve, and there must be a Jam Master to enforce them by group acceptance (which may involve a quiet comment at the end of the jam, up to a direct intervention).

No talking by others during the tune unless you're in a pub, etc.

Rythmn is not left to beginners, though the person who is learning can start a tune out at his speed, and expect that before it is over, someone will call out "At Speed", and the "normal" speed will take over.

All are expected to pick a song, so be prepared to pick at least ten tunes that evening, being prepared to change your pick if someone picks it before you. (It is better so say "Pass" than fumble around).

Learn the names of the tunes and bring a recorder if you can't write.

Very few of the songs or tunes we do require rythmn, so drums of any kind are usually not welcome unless you play another instrument.

"Strummers" must understand the concept of following the lead, and that they are not driving the tune but allowing others to move with the tempo, so that all are together. The worst kind of jam has a hundred strummers and one lead. You have three delicate pick-up notes and suddenly the place sounds like a Polish Flamenco dance, with each "strummer" (ie "parlor player") trying to play at frantic or loud, unlearned or undisciplined pace. This is OK for songs like "You Are My Sunshine" but death on a waltz or a jig.

Children are welcome, but must be under control and not allowed to run wild (there are expensive instruments, and on occasion I point out that "My Martin is worth more than the life of your child").

I could go on but the essential concept is to create an environment in which the good players will keep coming back, so that the novice players can learn from them and the experience of playing in a group setting.