I go to several sessions where we only play American Old Time tunes (and a few songs). I've been to other Old Time sessions here in the UK and also in the USA, and they mostly follow a similar format.

There is a set of "rules" which are implied but rarely stated. These are:
1. A key is decided upon, which is stayed in for a number of tunes until people decide on a new key. Sessions often stay in one key all night. This seems very strange to newcomers, but the reason is that fiddlers and banjo players often re-tune their instruments in a traditional way for different keys (usually D,G,A,C or occasionally modal A). There are a LOT of tunes in each key, which people gradually learn.
2. Anyone can start a tune and has the prerogative of setting the pace and deciding how long it continues. Tunes can continue for many iterations, which is great for people who've never heard it before. It's a great learning process, and a tune can develop a fine "groove" when no-one wants it to end. I've seen great tunes continue for 20 minutes... quite sexy really! You develop a sense of how long is appropriate, and people are generally very tolerant of beginners doing a tune their way - we've all been there!
3. The only percussion everyone tolerates is from the shuffling feet of a "flatfoot" dancer, preferably without taps. A good dancer can really lift a tune... Absolutely NO bodhrans, shaky eggs, tambourines, washboards, bones or drum kits. Everyone must be able to hear the tune as clearly as possible.
4. Traditional American string instruments are encouraged - usually fiddles, banjos, guitars, mandolines, an upright bass. No amplification. No squeezeboxes - unless it's a Cajun session.
5. Everyone contributes to the sound of the tune. We don't take solos (that's Bluegrass!) We listen to what people are doing and contribute to try and improve the overall sound. Play the version of the tune that the person starting it plays. Most of my repertoire of tunes has been picked up in sessions like this, and improved by listening to the old and great traditional players on record.

It's a great sound ..... Yahoo!
Ray