ground rules - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Basic procedures of conduct, as in The press secretary sets the ground rules for all of the President's press conferences. The term comes from baseball, where it refers to specific rules for a particular ballpark, which are based on special conditions such as a very high outfield fence or a field obstruction of some kind. It began to be transferred to more general use in the mid-1900s.