If you’ve ever stared at a contracts question on a practice MBE and thought, “Wait — does it matter how they accepted?” — yes. It absolutely does. The distinction between bilateral contracts and unilateral contracts is one of those foundational concepts that shows up quietly in formation questions and can completely change your analysis if you get it wrong.

Let’s break it down clearly so you never mix them up again.

What Is a Bilateral Contract?

A bilateral contract is a promise exchanged for a promise. Both parties make commitments at the moment of formation. The offeror says, in effect, “I promise to do X if you promise to do Y.” The offeree accepts by promising to perform — not by actually performing.

Think of it this way: you and a contractor sign a home renovation agreement. You promise to pay $10,000. The contractor promises to complete the work by a certain date. Neither party has done anything yet. But a contract exists the moment those promises are exchanged. That’s bilateral.

The vast majority of contracts you’ll see on the MBE are bilateral. When you’re unsure which type you’re dealing with, the modern rule applies a presumption in favor of bilateral contracts. More on that in a moment.

What Is a Unilateral Contract?

A unilateral contract is a promise exchanged for performance. The offeror doesn’t want a return promise — they want the actual, completed act.

The classic example: “I’ll pay you $100 if you walk across the Brooklyn Bridge.” The offeror isn’t asking you to promise to walk across. They’re asking you to walk across. Acceptance happens only when you complete the requested performance. Until then, there’s no contract.

This distinction matters enormously for acceptance timing. In a bilateral contract, acceptance is complete the moment the offeree makes the return promise. In a unilateral contract, acceptance isn’t complete until the offeree finishes the act.

The Bilateral vs. Unilateral Contracts Elements You Need to Know

Here’s the core framework laid out cleanly:

Bilateral contract elements:

Unilateral contract elements:

One thing that trips students up: in a unilateral contract, the offeree has no obligation to perform. They can walk away at any point before finishing. But once they start, something important kicks in.

The Irrevocability Rule — A Major MBE Trap

Here’s where bilateral vs. unilateral contracts on the MBE get genuinely tricky.

The general rule is that an offeror can revoke an offer any time before acceptance. In a bilateral contract, that window closes the moment the offeree makes the return promise. Clean and simple.

In a unilateral contract, though, acceptance doesn’t happen until the act is complete. That means, technically, the offeror could revoke mid-performance. And historically, courts allowed this. Harsh result, right?

Modern contract law fixed that. Once the offeree has begun performance, the offer becomes irrevocable. The offeror must give the offeree a reasonable opportunity to complete the act. Critically, though, mere preparation does not count as commencement of performance. Preparation gets you nothing. You have to actually start doing the thing.

So imagine this MBE-style hypothetical: An offeror posts a notice saying, “I’ll pay $500 to anyone who finds and returns my lost dog.” A neighbor spends two hours preparing — printing flyers, mapping search routes, buying dog treats. Before the neighbor starts searching, the offeror calls and says, “Never mind, I found the dog.” Has the offer been revoked?

Yes. Preparation is not performance. The neighbor hadn’t begun the requested act (searching), so the offer was still revocable. If the neighbor had actually been out searching when the revocation came through, the analysis would be different.

The Ambiguity Problem and the Modern Presumption

What happens when the offer doesn’t clearly specify whether the offeror wants a promise or an act?

This is where the modern presumption does real work. Under the Restatement (Second) of Contracts, when it’s ambiguous whether an offer calls for a promise or performance, courts presume the offer invites acceptance by either — and treat it as bilateral.

Why? Because bilateral contracts protect both parties earlier. The offeree gets the security of a formed contract without having to complete performance first.

On the MBE, if you see an offer that could be read either way, lean bilateral unless the language clearly and specifically demands completion of an act as the only method of acceptance.

How This Plays Out in MBE Questions

Let’s look at how these issues get tested. Typical MBE bilateral vs. unilateral contracts questions will involve:

1. Timing of acceptance. An offeror tries to revoke after the offeree has already made a return promise (bilateral — too late to revoke) or after the offeree has begun performance (unilateral — also too late to revoke under the modern rule).

2. Whether a contract was formed at all. If the offer required completed performance and the offeree only promised to perform, no contract exists yet. The offeror can still walk away.

3. The preparation trap. The offeree did preparatory acts but hadn’t started the actual performance. Revocation is still valid.

4. Ambiguous offers. The question tests whether you know to apply the bilateral presumption when the offer’s terms don’t clearly specify.

Watch for language in the offer. “I will pay you $200 if you paint my fence” — that’s likely unilateral. “I will pay you $200 if you promise to paint my fence by Friday” — that’s bilateral. The word “promise” is a signal. So is demanding a specific act with no mention of a return commitment.

Performance as Acceptance in Both Contract Types

One more nuance worth flagging. Performance can constitute acceptance of either a bilateral or a unilateral offer. In a bilateral contract, starting performance can sometimes imply the return promise. In a unilateral contract, completing performance is the acceptance. The question is always: what did the offer invite?

Also note that under the UCC, the rules shift slightly. A seller can accept a buyer’s offer by shipping conforming goods — or even non-conforming goods with appropriate notice. The UCC is more flexible about how acceptance can occur. But the bilateral/unilateral framework still matters for understanding when and how a contract is formed.

FlashTables

FlashTables is a set of professionally formatted two-column PDF rule tables covering all seven MBE subjects — 704 rules total, organized by the official NCBE Subject Matter Outline. The bilateral vs. unilateral contracts rule, including the irrevocability trigger and the modern bilateral presumption, is one of the 106 Contracts rules laid out side-by-side in the table. Whether you’re a law student building your contracts outline or a bar taker drilling active recall in the final weeks before the MBE, the tables give you every rule in a clean, testable format at getflashtables.com.

Key Takeaways: What to Memorize

Before you move on, lock in these rules:

Get these rules cold. Contracts formation questions are everywhere on the MBE, and bilateral vs. unilateral is one of the cleaner distinctions you can master — if you actually take the time to do it right.