Why etiquette matters
Tennis etiquette isn't about being fancy or pretentious. It's about making the game work when there's no referee. You call your own lines, you manage your own behaviour, and you share courts with other players. Without some agreed standards, it falls apart.
Most etiquette is common sense. Don't be a jerk. Don't cheat. Don't make others uncomfortable. The specifics are just applications of that basic principle.
Line calls
In social tennis, you call the lines on your side. Your opponent calls theirs. The system works on trust.
Call balls out clearly and immediately. "Out!" not a vague hand gesture three seconds later. If you're not sure, play it as in. If you make a wrong call, correct it and give the point to your opponent.
Don't call balls on your opponent's side (that's their job) and don't argue their calls. If you think someone is consistently cheating, finish the match and don't play them again. Life's too short.
Court behaviour
Don't talk or make noise during points. It's distracting and rude. Save conversations for changeovers.
Between points, the server calls the score loudly enough for everyone to hear. If you're unsure of the score, ask before the point starts, not after.
Control your temper. Nobody likes the guy throwing rackets and swearing at himself. It's uncomfortable for everyone else and embarrassing in hindsight.
Balls from other courts roll onto yours? Stop play, return the ball gently, and replay the point. Don't smash it back while they're mid-rally.
Sharing courts
If courts are busy and people are waiting, limit your session. An hour is typical. Thirty minutes if it's really packed.
Walk behind courts only when a point isn't in progress. Wait at the back fence until there's a break, then move quickly.
If someone's ball lands on your court, wait for a pause in your own play, then roll it back. Making them wait is rude; interrupting your own point is unnecessary.
Warming up
Both players get time to warm up, so don't race through it. Hit cooperatively (to each other, not away from each other). Let your opponent practice serves too.
Drop shots and trick shots during warm-up are bad form. Save them for match play.
After the match
Shake hands at the net regardless of the result. Say something genuine, like "Good match" or "Thanks for the game." If you lost badly, don't make excuses. If you won easily, don't gloat.
Pick up all balls before leaving. Take your rubbish. Leave the court in good condition.
The underlying principle
Treat others as you'd want to be treated. Give the benefit of the doubt. Be generous when it costs you nothing. These principles apply beyond tennis, but tennis is a good place to practice them.