I would tentatively suggest that many of the bad manners raised above can be virtually eliminated by the leadership of the person responsible for running the session.

My sessions are a famously informal mix of songs and tunes, but I endeavour to ensure that everyone has their fair crack of the whip, regardless of experience or musical ability. My regulars are aware of this.

If singers would like to sing without the benefit of accompaniment, they politely request it before they start the song. If people are getting too rowdy, or talking through a quiet song, a quiet word in the shell-like usually does the trick. And if somebody is playing loudly over everything (even in the wrong key) I have been known to take that person quietly to one side and politely ask if they might consider backing off a little.

The borrowing and swapping of instruments is commonplace, and should be at the discretion of the owner of said instrument. There is little I can do about this. In fact, I'm a major culprit of said crime.

This approach tends to keep good order, promotes a varied session, and generally, nobody gets upset! I would hope that my regulars agree with this. They keep coming back....