In the law of contracts, the mirror image rule states that an offer must be accepted exactly without modifications. An attempt to accept the offer on different terms instead creates a counter-offer - which constitutes a rejection of the original offer.

In the United States, this rule has been altered with respect to merchants dealing under the Uniform Commercial Code. In such situations, an acceptance that does not match the terms of the offer is nonetheless effective. The terms of the acceptance that do not materially differ from the original offer will govern the contract as a whole.