Mirror Image Rule

The mirror image rule is a foundational principle of common-law contract formation: to form a binding contract, the acceptance must precisely mirror the terms of the offer. Any change, addition, or qualification, even a minor one, is treated as a rejection of the original offer plus a new counter-offer that the original offeror is free to accept or reject.

In California, the rule still controls non-goods contracts: real estate purchases, service agreements, employment contracts. Practitioners exchanging redlines should be aware that a "yes, but…" response is legally a rejection. The original offer dies; if the counter-offer is not accepted, neither side is bound. Time-sensitive deals can collapse on what looked like a friendly clarification.

Sales of goods are different, Cal. Comm. Code §2207 displaces the mirror image rule for transactions in goods, allowing a definite acceptance to form a contract even with additional or different terms, subject to its own rules about which terms control. Knowing which body of law governs the deal is the threshold question whenever an acceptance does not perfectly track the offer.