You’re staring at an MBE question about whether a contract was properly formed, and you freeze. Is this a goods question or a services question? Does the mirror image rule apply here, or is it that UCC battle of the forms thing? And wait—do I need to know the price term or not?

If common law vs UCC contract formation trips you up on practice questions, you’re not alone. The MBE loves testing the distinctions between these two frameworks because they govern formation differently in ways that directly affect whether a contract exists. Master these differences, and you’ll immediately recognize which rule applies and how to analyze the fact pattern.

Why the Common Law vs UCC Distinction Matters on the MBE

The threshold question in any Contracts MBE question is: which law governs?

Common law governs contracts for services, real estate, and employment. UCC Article 2 governs contracts for the sale of goods—meaning all things movable and tangible at the time of identification to the contract. Think cars, furniture, crops, and electronics. Not real estate. Not legal services. Not employment agreements.

The MBE will test mixed contracts (goods plus services) using the predominant purpose test. If a contract involves both installing a security system and selling the equipment, ask: what’s the main point of this deal? If it’s primarily about the goods, UCC applies. If it’s primarily about the service, common law applies.

Why does this matter? Because once you identify which law governs, you know which formation rules to apply—and they’re different in critical ways.

Required Terms: Common Law Demands Certainty, UCC Fills the Gaps

Under common law, an offer must include all essential terms: parties, subject matter, price, and time of performance. Miss one of these, and you don’t have a valid offer. The law assumes parties need certainty before they’re bound.

Real estate contracts are even stricter. You must identify the specific land and state the price. “I’ll sell you my house for a fair price” isn’t an offer under common law—it’s too vague to enforce.

The UCC takes the opposite approach. Under UCC Article 2, the only required term is quantity. That’s it. If the parties leave out price, delivery location, or time of performance, the UCC supplies reasonable terms through its gap filler provisions:

Here’s how this plays out on the MBE. Imagine this fact pattern:

Buyer emails Seller: “I’ll purchase 500 widgets from you.” Seller replies: “Agreed.” Nothing else is discussed. Is there a contract?

Under common law? No. No price, no delivery terms, no time for performance—too indefinite.

Under the UCC? Yes. Quantity is specified (500 widgets), and the UCC fills in the rest. The court would supply a reasonable price and reasonable delivery terms. Contract formed.

The examiners love this distinction. They’ll give you a sparse agreement and ask whether it’s enforceable. Identify whether it’s goods or services, then apply the right standard.

The Mirror Image Rule vs. Battle of the Forms

This is where students lose the most points, because the common law and UCC rules are polar opposites.

Common Law: Mirror Image Rule

Under common law, acceptance must be the mirror image of the offer. Any change in terms—no matter how minor—is a counteroffer, not an acceptance. The counteroffer terminates the original offer.

Suppose a law firm offers to hire an associate for seventy-five thousand dollars per year. The associate responds: “I accept, but I’d like eighty thousand dollars.” That’s not acceptance. It’s a counteroffer. The original offer is dead. The firm can walk away.

This rule is unforgiving. Even adding a term like “I accept, and I’d also like two weeks of vacation” kills the original offer under strict common law.

UCC: Battle of the Forms (§2-207)

The UCC scrapped the mirror image rule because commercial parties exchange pre-printed forms that never match perfectly. Under UCC Section 2-207, a definite expression of acceptance operates as acceptance even if it contains additional or different terms.

The analysis then splits depending on whether both parties are merchants (professionals in the business of buying or selling goods):

Between merchants: Additional terms automatically become part of the contract unless:

  1. The offer expressly limits acceptance to its terms
  2. The new terms materially alter the contract
  3. The offeror objects within a reasonable time

Between non-merchants (or if one party isn’t a merchant): Additional terms are mere proposals. They don’t become part of the contract unless the offeror expressly agrees.

Here’s an MBE-style hypo:

Retailer sends a purchase order for 1,000 coffee mugs at three dollars each. Manufacturer responds with an acknowledgment form agreeing to the quantity and price, but adding an arbitration clause. Both parties are merchants. Is there a contract, and does it include arbitration?

Yes, there’s a contract—the acknowledgment was a definite expression of acceptance despite the added term. Whether the arbitration clause is included depends on whether it materially alters the deal. Arbitration clauses typically do materially alter contracts, so unless Retailer expressly agreed, the arbitration term likely doesn’t become part of the contract. But the contract itself still exists.

Contrast that with common law, where Manufacturer’s response would be a counteroffer, and no contract would exist until Retailer accepted the new terms.

Modification Rules: Consideration Required or Not?

The common law and UCC also diverge on contract modifications.

Common law follows the pre-existing duty rule: a modification requires new consideration. If you’re already obligated to paint my house for five thousand dollars, your promise to do the same work isn’t valid consideration for my promise to pay you six thousand dollars. You’re not doing anything you weren’t already legally required to do.

There are exceptions (new or different consideration, unforeseen circumstances, honest dispute about the duty), but the baseline rule is: modifications need fresh consideration.

UCC Section 2-209 throws this out entirely. Under the UCC, a contract modification requires only good faith—no new consideration needed.

Example:

Supplier agrees to deliver 10,000 units of steel to Manufacturer for one hundred thousand dollars. After signing, global steel prices spike. Supplier asks Manufacturer to pay one hundred twenty thousand dollars. Manufacturer agrees in writing. Is the modification enforceable?

Under common law, probably not—Supplier is only doing what it already promised to do, so there’s no new consideration.

Under the UCC, yes—as long as the modification was made in good faith (not through coercion or bad faith threats), it’s enforceable without new consideration. The price spike and Manufacturer’s voluntary agreement suggest good faith.

One wrinkle: if the contract as modified falls within the Statute of Frauds (generally, goods priced at five hundred dollars or more), the modification must be in writing. But no new consideration is required.

The UCC Firm Offer: Irrevocability Without Consideration

Common law requires consideration to make an offer irrevocable. That’s why option contracts exist—you pay me ten dollars to keep my offer to sell you my car open for 30 days.

The UCC created an exception: the firm offer rule under Section 2-205.

A merchant’s signed written offer to buy or sell goods is irrevocable for the stated time, or up to three months if no time is stated—and no consideration is required.

Three requirements:

  1. The offeror must be a merchant (someone who deals in goods of that kind)
  2. The offer must be in a signed writing
  3. The writing must give assurance that the offer will be held open

If a car dealer signs a written offer to sell you a vehicle and states “this offer will remain open for two weeks,” that’s a firm offer. The dealer can’t revoke it during those two weeks, even though you paid nothing to hold it open.

This only applies to goods. A lawyer offering to represent you for a flat fee can revoke anytime before you accept, even if the offer is in writing, because legal services are governed by common law.

Acceptance by Performance: Unilateral Contracts Under Both Frameworks

Both common law and the UCC recognize that performance can constitute acceptance, but there’s a critical protection under common law for unilateral contracts.

A unilateral contract is a promise exchanged for performance (not for another promise). Classic example: “I’ll pay you five hundred dollars if you paint my fence.”

Under common law, once the offeree begins performance, the offer becomes irrevocable. The offeror can’t snatch the offer away while you’re halfway through painting the fence. However, mere preparation (buying paint and brushes) doesn’t start performance—you must actually begin the requested act.

The UCC doesn’t have a separate rule here, but the same principle applies. If a buyer offers to pay for goods upon delivery and the seller begins shipment, the offer is irrevocable.

The MBE tests whether the offeree has actually commenced performance or merely prepared. Starting to paint the fence? Irrevocable. Buying supplies? Not enough.

Putting It All Together: How to Spot the Issue on the MBE

When you see a Contracts question, follow this checklist:

  1. Identify the subject matter: Goods (movable, tangible) or something else?
  2. Apply the right law: UCC Article 2 for goods, common law for everything else
  3. Check required terms: Common law needs all essential terms; UCC only needs quantity
  4. Analyze acceptance: Mirror image rule (common law) or battle of the forms (UCC)?
  5. Evaluate modifications: New consideration required (common law) or just good faith (UCC)?

Most wrong answers on formation questions stem from applying common law rules to goods contracts or vice versa. The examiners know students mix these up under time pressure.

What You Need to Memorize

Here’s your takeaway for common law vs UCC MBE questions:

Governing Law: UCC Article 2 = goods (movable, tangible). Common law = services, real estate, employment. Mixed contracts = predominant purpose test.

Required Terms: Common law requires parties, subject matter, price, and time. UCC requires only quantity—gap fillers supply the rest.

Mirror Image Rule (Common Law): Acceptance must exactly match the offer. Any change = counteroffer.

Battle of the Forms (UCC §2-207): Acceptance with additional terms still forms a contract. Between merchants, additional terms become part of the contract unless they materially alter it, the offer limits acceptance, or offeror objects.

Modification: Common law requires new consideration. UCC requires only good faith (no new consideration needed).

Firm Offer (UCC §2-205): Merchant’s signed written offer to buy/sell goods is irrevocable up to three months—no consideration required.

If you want all the contract formation rules organized in a format built for active recall, the FlashTables Contracts table covers these distinctions in a two-column structure that makes comparison easy. It includes 106 rules across the entire NCBE Contracts outline, with each element broken down exactly as you need it for the MBE.

The key to mastering common law vs UCC contract formation isn’t memorizing every nuance—it’s training yourself to immediately categorize the transaction (goods or not goods?) and then applying the right framework. Do that consistently, and these questions become some of the fastest points you’ll earn on test day.