The contract gives the law.

Explanation

The contract gives the law to the transaction — the terms agreed upon by the parties, not external default rules, govern the rights and obligations arising from their agreement.

Where this applies

• Two parties agree in their contract that disputes will be resolved by arbitration in Chicago, and the court enforces that term because the contract gives the law to their relationship.
• A lease specifies that the tenant must pay utilities directly, overriding the local custom of landlord-paid utilities — the contract's own terms govern.
• A service agreement states that payment is due within 15 days rather than the standard 30-day trade term, and the court applies the 15-day deadline because the parties' agreement is their law.