Four Corners Rule
four corners rule n: a rule holding that if a document (as a contract, deed, or will) appears on its face to be complete no outside evidence may be used to challenge it NOTE: The number of states that accept the four corners rule is in decline.
Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.