A contract has three parts: an offer, an acceptance, and consideration. The offer is a proposal with specific terms, the acceptance is an unconditional agreement to those terms, and consideration is the thing of value each side gives up in the exchange. Remove any one of the three parts of a contract and what you have is a promise a court will not enforce.
The Offer
A contract starts when one party proposes a deal with clear, specific terms. “I’ll sell you my push mower for $150” qualifies because it identifies the item, the price, and who’s involved. A vague statement like “I might sell some stuff” does not. The person making the offer controls its terms and can attach whatever conditions they want: a deadline, a required method of acceptance, or any other specifics.
One common point of confusion. Most advertisements, items on store shelves, and prices on menus are not offers. Lawyers call them invitations to treat, meaning the business is inviting you to make an offer, not the other way around. When you bring an item to the register, you are the one proposing the purchase, and the store can accept or decline. The classic exception is Carlill v. Carbolic Smoke Ball Co., where a company’s advertisement promising £100 to anyone who used its product and still got the flu was held to be a binding offer because its terms were unusually specific and the company had deposited money in a bank to back the promise.1Justia. Carlill v Carbolic Smoke Ball Co
How Long an Offer Stays Open
The person who makes an offer can pull it back at any time before the other side accepts, as long as they communicate the revocation. Once the other party rejects the offer or responds with different terms, the original offer dies and cannot be revived later. If no deadline is stated, the offer stays open for a reasonable period given the circumstances. Courts look at the nature of the deal, the speed of prior communications, and trade customs to figure out what reasonable means.
The main exception is an option contract. If the person receiving the offer pays something, even a small amount, to keep it open for a set period, the offer becomes irrevocable during that window. This is common in real estate, where a buyer might pay a few hundred dollars for the right to accept a purchase offer within 30 days.
The Acceptance
Acceptance is the unconditional yes to the offer’s terms, creating what courts call a meeting of the minds. Under the mirror image rule, the acceptance must match the offer exactly.2Legal Information Institute. Mirror Image Rule Change a single term and the response stops being an acceptance. It becomes a counter-offer that kills the original deal.
The 1840 English case Hyde v. Wrench is still the standard illustration. A seller offered his farm for £1,000. The buyer responded with £950. The seller refused, and the buyer then tried to accept the original £1,000 price. The court said no. The counter-offer had destroyed the original offer, and there was nothing left to accept. That rule still holds in common-law contract disputes today.
The UCC Exception for Sales of Goods
The mirror image rule works well for simple deals, but it caused chaos in commercial transactions where buyers and sellers routinely exchanged forms with slightly different boilerplate. The Uniform Commercial Code, adopted in some form by every state, loosened the rule for sales of goods. Under UCC Section 2-207, an acceptance that adds or changes minor terms can still function as a valid acceptance rather than a counter-offer. Between businesses, additional terms automatically become part of the contract unless they materially change the deal, the original offer expressly limited acceptance to its exact terms, or the offeror objects within a reasonable time. This is a significant departure from the common-law approach and catches many people off guard.
How Acceptance Is Communicated
Acceptance must be communicated to the offeror through whatever method the offer specifies, or through any reasonable means if the offer is silent on the point. That can be a signed document, a verbal statement, an email, or even conduct. If a painter starts painting your house after you propose a price, they have accepted by performance.
When acceptance is sent by mail, the mailbox rule adds a timing wrinkle. The acceptance takes effect the moment it is dropped in the mail, not when it arrives.3Legal Information Institute. Mailbox Rule The same logic applies to other communication methods where the message is irrevocable once sent. Parties can override this default by stating in the offer that acceptance is only effective upon receipt.
The Consideration
Consideration is the what’s-in-it-for-each-side element. It is the bargained-for exchange that separates an enforceable contract from a gift or a favor.4Legal Information Institute. Consideration Each party must give up something, or promise to do (or not do) something, in return for the other’s promise. Without this exchange, a court will generally refuse to enforce the agreement no matter how clearly both sides expressed their intent.
Consideration does not have to be cash. It can be a service, a physical item, a promise to act, or a promise not to act. The landmark case Hamer v. Sidway settled this. An uncle promised his nephew $5,000 if the nephew stopped drinking, smoking, and gambling until he turned 21. The nephew held up his end, and the court found that giving up his legal right to do those things was valid consideration, even though the uncle arguably received no direct financial benefit.5Justia. Hamer v Sidway
Courts generally do not care whether the exchange is lopsided. Selling a car worth $10,000 for $1 is technically supported by consideration, and courts will not second-guess the fairness of the bargain in most situations. What they do care about is whether the consideration is real and current.
What Does Not Count as Consideration
Two situations trip people up most often. Past consideration, meaning something you already did before the promise was made, does not support a new contract. If your neighbor mows your lawn without being asked and you later promise to pay $50 for the work, that promise is generally unenforceable because the mowing was not part of a bargain. A pre-existing duty also fails as consideration. If a contractor is already obligated to finish a project by December, promising to finish by December in exchange for a bonus adds nothing new to the deal.
The Fairness Exception: Promissory Estoppel
Sometimes fairness demands enforcement even when consideration is missing. Under the doctrine of promissory estoppel, a court can enforce a promise if the person receiving it reasonably relied on it to their detriment, the person making it should have foreseen that reliance, and enforcing the promise is the only way to prevent injustice.6Legal Information Institute. Promissory Estoppel A classic example: an employer promises a job, and the applicant sells their house and relocates in reliance on that promise. Even without formal consideration, a court may hold the employer to the promise or award damages.
Why the Three Parts Alone Are Not Always Enough
Offer, acceptance, and consideration get you a contract in theory. A few other requirements decide whether a court will actually enforce it in practice.
Both parties must have legal capacity, which in most states means being at least 18 and mentally competent. Contracts signed by minors or by someone who lacked the mental capacity to understand the terms are voidable by that party. The deal itself must be legal: a contract to commit a crime or violate public policy is void from the start. And both sides need genuine mutual assent to the same terms. If each party attached a completely different meaning to a key term and neither had reason to know about the misunderstanding, there was no true meeting of the minds and no contract.
Some contracts also must be in writing under a rule called the Statute of Frauds. Real estate transactions, agreements that cannot be performed within one year, promises to pay someone else’s debt, prenuptial agreements, and sales of goods priced at $500 or more all fall into this category.7Legal Information Institute. UCC 2-201 Formal Requirements – Statute of Frauds A signed email, text message, or even a napkin with the essential terms can satisfy the requirement, but without some tangible writing a court will typically refuse to enforce the agreement regardless of what the three parts look like.
Finally, even a contract that has all three parts and clears every other requirement can be challenged if it was formed through duress, fraud, material misrepresentation, or unconscionable terms imposed through an unfair bargaining process.8Legal Information Institute. Unconscionability The party who was pressured, lied to, or trapped by one-sided terms can usually walk away from the deal.