ground rule
ground rule {n.}
1. A rule in sports that is made especially for the grounds or place where a game is played. — Usually used in the plural.
There was such a big crowd at the baseball game, that the ground rules of the field were changed in case a ball went into the crowd.
2. A rule, usually not written, of what to do or how to act in case certain things happen. — Usually used in the plural.
When you go to a new school, you don't know tire ground rules of how you are supposed to behave.
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Source: A Dictionary of American Idioms
