Reggie, you raise an interesting question. It is certainly true to say that some people are predjudiced against certain instruments in particualr situations but why should that be? How many Irish instruments are there in an Irish session? The bozouki? Even the banjo? ... Where do we stop the clock - perhaps even the fiddle doen't belong...

I think that it is fair to say that some instruments are unlikely to fit in to certain types of music but there often are players who can make the "unexpected" work... and I believe people should be open to new instruments... unless of course the aim of a venu is to be purely tradational, which comes back to the traditional at what point in history...

As far as I am aware, sessions and Jam sessions are one and the same. They are pretty variable things and a lot depends on the people that are playing in the session (and the attitude of a session leader, landlord of a pub... if as and where such people exist). When it works, playing in a session, joining in with other players is IMO the most enjoyable musical experience going and you can get "carried along" with the music. There are no hard and fast rules but as with most aspects of life, it only takes one person to ruin it for everybody else.

Jon