Mirror Image Rule in Contract Law: Bar Exam Quick Guide

Introduction

In US contract law, the mirror image rule says an acceptance must match the offer exactly. If the offeree changes a term or adds a new one, that response is not an acceptance—it’s a counteroffer. This rule applies to common law contracts (services, real estate, employment, etc.). For sales of goods, Article 2 of the Uniform Commercial Code (UCC) softens the rule through Section 2-207, often called the battle of the forms. Timing also matters: under the mailbox rule, a properly dispatched acceptance is effective when sent (with key exceptions).

If you’re preparing for the bar exam, you’ll see these concepts in both MBE questions and essays. This guide breaks them down with clear examples and case references commonly taught in US courses.

What You'll Learn

• What the mirror image rule requires under common law
• How to tell a true acceptance from a counteroffer or mere inquiry
• When the mailbox rule makes an acceptance effective upon dispatch
• How UCC 2-207 changes the strict mirror image approach for goods
• What counts as a “material alteration” under UCC 2-207
• How to analyze mixed communications (emails, letters, forms) on the exam
• Case touchpoints: Ardente v. Horan; Minneapolis & St. Louis Ry. v. Columbus Rolling-Mill; Morrison v. Thoelke; Brown Machine v. Hercules; Step-Saver v. Wyse

Core Concepts

Mirror Image Rule at Common Law

• Exact match required: Acceptance must mirror the offer. Any added, different, or conditional term is a counteroffer and operates as a rejection of the original offer.
• Conditional acceptance: Under Restatement (Second) of Contracts §59, a reply that purports to accept but is made conditional on the offeror’s assent to additional or different terms is a counteroffer.
• Requests vs conditions: Restatement §61 allows an acceptance to include a request for a change or addition, as long as the acceptance is not made to depend on that request. A “grumbling acceptance” (“I accept, but I wish the price were lower”) is still an acceptance if it does not impose a new condition.
• Offeror controls the terms and method: The offer can specify the exact terms and how acceptance must be made. Failing to comply may prevent formation.
• Counteroffer effect: A counteroffer (Restatement §39) rejects the original offer, unless the offeror revives it.

Practical takeaway: Read the offeree’s response closely. If acceptance is tied to a new term (“I accept if you include X”), that’s a counteroffer. If the offeree accepts and merely asks (“Could you also include X?”), that’s typically a valid acceptance.

Mailbox Rule and Timing of Acceptance

• Dispatch rule: Restatement §63 provides that an acceptance is effective when properly dispatched (mailed or otherwise sent in an authorized manner), even if it never reaches the offeror, unless the offer states otherwise.
• Exceptions:
  • Option contracts: Under §63(b), an acceptance under an option contract is effective upon receipt, not dispatch.
  • Method and address: The acceptance must be properly addressed and use a reasonable method. If the offer prescribes a specific method, use it.
  • Revocation timing: A revocation is effective upon receipt. If the offeree mails an acceptance before receiving a revocation, the contract is formed.
• Historical origin: The doctrine traces back to the English case Adams v. Lindsell. US courts follow the principle (see, for example, Morrison v. Thoelke, Fla. Dist. Ct. App. 1963).

Practical takeaway: On timing questions, write out the sequence. If a proper acceptance is sent before a revocation is received, the contract is formed (unless it’s an option).

UCC Article 2: Section 2-207 (Battle of the Forms)

For sales of goods, strict mirror-image matching is not required.

• Acceptance with additional terms: A definite and seasonable expression of acceptance operates as an acceptance even if it contains additional or different terms, unless acceptance is expressly made conditional on assent to those terms (2-207(1)).
• Additional terms between merchants: Additional terms usually become part of the contract unless:
  1. The offer expressly limits acceptance to its terms,
  2. The new term materially alters the contract, or
  3. The offeror objects within a reasonable time (2-207(2)).
• Different terms: Jurisdictions vary. Many courts apply the “knockout rule” (conflicting terms drop out and UCC gap-fillers apply). Others treat different terms like additional ones or follow a “first shot” or “last shot” approach depending on facts.
• Contract by conduct: If writings don’t form a contract but the parties perform, a contract exists under 2-207(3). Terms consist of those on which the writings agree, plus UCC gap-fillers. Conflicting terms are often knocked out.

Material alteration examples commonly include: surprise arbitration clauses, broad indemnity provisions, disclaimer of standard warranties, or significant limitation of remedies. See cases like Brown Machine, Inc. v. Hercules, Inc. (Mo. Ct. App. 1989) and Step-Saver Data Systems, Inc. v. Wyse Technology (3d Cir. 1991).

Practical takeaway: In goods cases, identify whether both parties are merchants, whether the acceptance was expressly conditional, and whether added terms materially change the deal.

Key Examples or Case Studies

• Exact acceptance vs counteroffer (car sale):

  • Offer: “I’ll sell my car for $5,000.”
  • Response A: “I accept, if you include new tires.” This is a counteroffer under the mirror image rule.
  • Response B: “I accept. Could you include new tires?” This is typically an acceptance with a nonbinding request (valid contract at $5,000 without new tires unless the seller agrees to add them).
  • Case touchpoint: Ardente v. Horan (R.I. 1976) found no contract where the buyer’s “acceptance” was conditioned on receiving certain items of furniture; the added condition made it a counteroffer.

• Counteroffer kills the offer (quantity change):

  • The offeree’s attempted “acceptance” that changes quantity or price is a counteroffer and rejects the original. See Minneapolis & St. Louis Ry. Co. v. Columbus Rolling-Mill Co., 119 U.S. 149 (1886).

• Mailbox rule in action:

  • Timeline: Monday, Seller mails offer. Tuesday, Buyer mails acceptance. Wednesday, Seller calls to revoke. The contract formed on Tuesday when the Buyer mailed the acceptance (assuming proper dispatch). Revocation on Wednesday is too late.
  • US application: Morrison v. Thoelke (Fla. Dist. Ct. App. 1963) applied the dispatch rule; Restatement §63 states the general rule. Note that Adams v. Lindsell is often cited as the historical starting point.

• UCC 2-207 additional terms (seller’s indemnity clause):

  • Facts: Seller’s acknowledgment accepts Buyer’s purchase order but adds a broad indemnity clause. The acceptance is not expressly conditional on Buyer’s assent.
  • Result: Contract on Buyer’s terms; the indemnity clause likely does not become part of the contract if it materially alters the deal or if Buyer objects. See Brown Machine, Inc. v. Hercules, Inc. (Mo. Ct. App. 1989).

• UCC 2-207 and box-top or post-sale terms:

  • Facts: Parties reach agreement by phone or purchase order; the goods arrive with box-top terms adding warranty disclaimers or arbitration.
  • Result: Many courts hold the contract formed earlier; later box-top terms are proposals that often do not become part of the contract, especially if they materially alter. See Step-Saver Data Systems, Inc. v. Wyse Technology (3d Cir. 1991). Some courts address conflicts using the knockout rule (e.g., Ionics, Inc. v. Elmwood Sensors, Inc., 1st Cir. 1997).

Practical Applications

• Quick exam framework:

  1. Identify the governing law. Is the subject a service/real estate (common law) or sale of goods (UCC Article 2)? For mixed contracts, apply the predominant purpose test.
  2. Pin down the offer. List the exact terms and any required method of acceptance.
  3. Read the response carefully. Is it an unconditional “yes,” a request, or an “I accept if…” condition?
  4. If common law: Any added or different term makes it a counteroffer. A mere request is fine if the acceptance does not depend on it.
  5. Check timing. Was acceptance properly dispatched before a revocation was received? If the offer is an option, acceptance is effective on receipt.
  6. If UCC Article 2 applies:
     • Was the acceptance expressly conditional on assent to new terms? If yes, no contract on the writings unless assent occurs; look to conduct under 2-207(3).
     • Are both parties merchants? If yes, additional terms may become part unless an exception applies.
     • Do the new terms materially alter the bargain? If yes, they do not become part without express assent.
     • For different/conflicting terms, consider your jurisdiction’s approach (knockout rule is common).
  7. If performance occurred despite mismatched forms, use 2-207(3) and fill gaps with the UCC.

• Common bar exam pitfalls:

  • Treating a conditional acceptance as a valid acceptance. If the acceptance depends on a new term, it’s a counteroffer.
  • Ignoring the offer’s stated method of acceptance. If the offer requires acceptance by a certain method, stick to it.
  • Forgetting the option contract exception to the mailbox rule.
  • Assuming all added terms in merchant-to-merchant exchanges become part. Screen for material alteration and timely objection.
  • Confusing a request for a change with a condition. Look for words like “if,” “only if,” or “provided that.”

• Practical red flags in real deals:

  • Surprise arbitration clauses, indemnity provisions, warranty disclaimers, or remedy limitations slipped into acknowledgments or invoices.
  • “Expressly conditional” language in acknowledgments: “This acceptance is expressly conditioned on your assent to the additional terms.”
  • Offers that limit acceptance: “Acceptance is limited to the terms of this offer.”

Summary Checklist

• Common law mirror image rule: Acceptance must match the offer. Added/different terms or conditions = counteroffer.
• Requests vs conditions: A request does not void acceptance unless acceptance depends on it (Restatement §61).
• Counteroffer effect: A counteroffer rejects the original offer unless revived.
• Mailbox rule: Acceptance effective on dispatch; not for option contracts (effective on receipt); revocation effective on receipt.
• Offer controls terms and method of acceptance; follow any stated method.
• UCC 2-207(1): Acceptance with additional terms can still form a contract unless expressly conditional.
• UCC 2-207(2): Between merchants, additional terms become part unless the offer limits acceptance, the term materially alters, or there’s a timely objection.
• UCC 2-207(3): Conduct can create a contract; conflicting terms often knocked out; use UCC gap-fillers.
• Material alteration examples: arbitration, broad indemnity, warranty disclaimers, significant remedy limits.

Quick Reference