Also known as Rule 0.5, due to its close relation to Rule Zero. Can be boiled down to one sentence:
Don't be a dick.
We all know about Rule Zero. Everyone playing an RPG is generally there to have fun, unless you're playing FATAL or something, and if the game isn't fun the players have the right to leave the game instead of wasting time and energy on a fool's errand. However "make the game fun" isn't the whole story; the GM is also expected to not handle the group with kid gloves or pander to players for fear of hurting their feelings, because that shit makes the experience hollow and usually causes games to devolve into communal fanfic-writing sessions. Thus, a Social Contract emerges, where the GM has an obligation to the group to run a challenging, fun game and the players have a collective obligation to not be That Guy and not give the GM shit for putting their special snowflake in a situation where they could conceivably fail.
Some terms of the Social Contract
- Don't give your players a no-win scenario, unless they really, really ask for it. Even the most hardcore of the classic Gygaxian deathtraps could be survived if the players have luck on their side and aren't blithering idiots, so Old School Roleplaying isn't an excuse.
- Magical realms. Avoid them. Unless your players know what they signed up for, of course.
- Don't build your character to be unstoppable at the expense of all else unless the rest of the group is doing the same and the GM has planned accordingly. You really don't want to be that one asshole who brings CoDzilla to a party of martial characters.
- Your character will fail at what they set out to do on occasion. Your character might even die. Tough shit.
- Magical realms. You should avoid them too.