The unwritten rule of tabletop Role Playing Games:
The Game/Dungeon Master has the right to veto anything any player says, he has the right to change any rule or make up his own, he need not explain why he choses to do these things. If players complain the GM may choose any of the following to do to the player; slap, call a dumbass, restrict snackage privileges and/or threaten injury to ingame character(be it through loss of xp, health, items or gold)
Player: Can I make my own species for my familiar
DM: Hells no! I'm gonna have to Rule 0 that one
Player: Bitch
DM: I swear I will cut you off of the mountain dew boy
Player: I'm sorry master.
The unwritten rule in tabletop role-playing games (such as Dungeons & Dragons) which grants the game master the right to suspend or override the published game rules whenever s/he deems necessary.
Similar to a house rule; however, rule zero may be invoked unilaterally, at any time and does not have to be agreed upon in advance by the players.
When abused, can lead to an inconsistent, confusing, or frustrating experience for the players.
Bob: I climb up the wall and into the wizard's castle, bypassing the defenses. I rolled a 20, so I succeed.
GM: The wall is magically warded against climbing. You fail.
Bob: But a 20 is an automatic success!
GM: Rule 0. You fail.
A rule of table top gaming. If the DM/GM is being an Ass/dumbass/idiot and/or abusing rule 0, the players can choose to override him by leaving his game and starting their own.
An alternate version involves Smacking the DM over the head with the Dungeon Master's Guide.
DM: The ghoul rolled a 28 On his check to resist your Trip attempt.
Player: How the Hell does a ghoul get a 28?
Dm: With His +8 grapple modifier.
Player: But it is a Strength check, not a grapple!!
DM: I am the DM, Under Rule 0 I say He uses his Grapple Modifier.
This is where Rule 0.5 Is invoked
Every player at table: Smacks DM upside head with books and Leaves game.
This is an unspoken rule in Table top RPGs. It is that the Game Master, or whoever is running the campaign, can simply say no to anything the other players suggest without explanation.
In other words, if someone explicitly says they don't like something or don't want to see something, that's all they'll see for an extended period of time. Examples include the Harambe meme, Ligma, and Philip Defranco being called "daddy".