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Parol Evidence Rule
The parol evidence rule generally prevents a party from introducing evidence of prior oral or written agreements that would contradict or add to the terms of a final written contract.
What it actually is
It is a substantive rule of contract law, not a rule of evidence, that applies when a court determines that a written agreement is intended to be the complete and final expression of the parties' agreement. In a case, the rule typically arises when one party tries to introduce outside statements or side agreements to change what the written contract says.
Why it matters in your case
If you are in a lawsuit over a contract, this rule can block you from arguing that the written contract does not tell the whole story, even if you had other promises made before signing. Practically, it means the written document often becomes the sole evidence of your agreement, so you must carefully review it before signing.
The common misunderstanding
Many people mistakenly think the rule applies to all evidence about a contract, but it only bars evidence of prior or contemporaneous agreements that contradict the written terms, not evidence about fraud, mistake, or conditions that never happened.
Get the actual rule
Definitions are orientation; rules are authority
This page explains the concept. When it matters to your case, read the rule that governs it in your court — we hold Texas, federal and Washington court rules word for word, and every state's official resources are on the state pages. If someone cites a case at you, check that it exists first.
Related
Terms that travel with this one
General explanation, not legal advice, and not specific to any state — procedures and deadlines vary by jurisdiction and court. If you can get a lawyer or free legal aid, do: every real option.