A more thorough explanation:

The collateral-contract doctrine is a legal principle that allows proof of a second agreement, usually oral, to be considered in a dispute concerning a written contract. This proof will not be excluded under the parol-evidence rule if the oral agreement is independent of and not inconsistent with the written contract. Additionally, the information in the oral agreement would not ordinarily be expected to be included in the written contract.

For example, if a person signs a contract to purchase a car and agrees to pay a certain amount of money, but the salesperson orally promises to include a free warranty, the oral agreement would be considered a collateral contract. The warranty is not included in the written contract, but it is not inconsistent with it. Therefore, the buyer could use the collateral-contract doctrine to enforce the salesperson's promise of a free warranty.

Another example could be a written contract for the sale of a house that includes a clause stating that the seller will leave all appliances in the kitchen. However, the seller orally promises to leave a new refrigerator as well. This oral agreement would be considered a collateral contract because it is independent of and not inconsistent with the written contract. The buyer could use the collateral-contract doctrine to enforce the seller's promise of a new refrigerator.

These examples illustrate how the collateral-contract doctrine can be used to enforce oral agreements that are not included in a written contract but are still valid and enforceable.