ground rule

[ground rule] {n.} 1. A rule in sports that is made especially forthe grounds or place where a game is played. - Usually used in theplural.

There was such a big crowd at the baseball game, that theground rules of the field were changed in case a ball went into thecrowd.

2. A rule, usually not written, of what to do or how to act incase certain things happen. - Usually used in the plural.

When yougo to a new school, you don't know tire ground rules of how you aresupposed to behave.