Friendly matches often become strained because weekend players don't know a couple of basic rules.
One rule states that you lose the point if you interfere with a ball that hasn't had a chance to bounce - even if it appears to be going out and you're standing behind the baseline.
To avoid possible confusion, always let the ball bounce before touching it.
Another much-misunderstood rule applies to what you can and cannot do with your racket when you are playing close to the net.
You are not allowed to reach over the net and hit the ball before it crosses to your side. You are allowed to follow through with your racket across the net, so long as your racket does not touch the net.
Knowing the basic rules is essential to playing any game but especially a leisure sport like tennis, where friendship is as important as competitiveness.