A contract exists when one party makes an offer, the other accepts it on the same terms, and each side gives something of value in exchange. Those three pieces — offer, acceptance, and consideration — are the structural minimum. Remove any one and you have a conversation, not an enforceable agreement.1Legal Information Institute. Contract
The Offer
An offer is a proposal from one person (the offeror) to another (the offeree) signaling a willingness to be bound on specific terms. To qualify as a legal offer, three things have to be true: the offeror must intend to be bound if the other side says yes, the terms must be clear enough for a reasonable person to understand, and the offer must actually reach the offeree.2Legal Information Institute. Offer
A common misconception: price tags, website listings, and newspaper ads are usually not offers. Lawyers call them invitations to treat. They invite the customer to make an offer, which the seller then accepts or rejects. The person at the register saying “I’ll take this” is the one making the offer. The store accepts by taking payment. The distinction matters because it decides who can walk away. If an ad were a binding offer, the store would owe a sale to everyone who showed up, even after inventory ran out.
How an Offer Ends
An offer doesn’t sit open forever. The offeror can revoke it any time before acceptance, so long as the revocation reaches the offeree first.2Legal Information Institute. Offer Revocation isn’t the only exit, though. An offer also terminates when:
- It expires. If the offer sets a deadline, it dies at the deadline. If none is stated, it lapses after a reasonable time, which depends on the circumstances. A verbal offer made in person generally needs to be accepted during the conversation. A mailed offer might survive for days.
- The offeree rejects it. Once you say no, the offer is gone. You can’t circle back the next day and accept.
- The offeree makes a counteroffer. A counteroffer rejects the original and proposes a new one at the same time. If someone offers to sell you a car for $10,000 and you respond with $8,000, the $10,000 offer no longer exists.
- Either party dies or loses capacity. The offer terminates automatically, with no communication needed.
There is one important exception. If the offeree pays the offeror to keep an offer open for a set period, that arrangement is an option contract. The offeror has received something of value in exchange for the promise not to revoke, so the offer stays open until the agreed deadline.2Legal Information Institute. Offer Option contracts show up often in real estate, where a buyer might pay $500 to lock in the right to purchase a property within 60 days.
The Acceptance
Acceptance is the offeree’s agreement to the terms of the offer. It can be spoken, written, or shown through conduct, but it has to be communicated in a way the offeror authorized or would reasonably expect.3Legal Information Institute. Acceptance Nodding along silently while someone describes a deal generally doesn’t count. Silence is not acceptance unless the parties have a prior relationship or an explicit agreement saying otherwise.
Acceptance Has to Match the Offer
Under traditional contract law, the acceptance has to mirror the offer. Change the price, the delivery date, or the quantity, and you haven’t accepted. You’ve made a counteroffer, which kills the original and puts a new proposal on the table.4Legal Information Institute. Mirror Image Rule This is where plenty of deals quietly fall apart. One side thinks they accepted, but because they tweaked a single term, they actually rejected the offer and proposed a different one.
The mirror image rule applies strictly to common-law contracts covering services, real estate, and employment. For contracts involving the sale of goods, the Uniform Commercial Code softens it. Under UCC Section 2-207, an acceptance that adds or changes terms can still operate as a valid acceptance rather than a counteroffer. Between merchants, those extra terms may even become part of the contract unless they materially change the deal, the original offer limited acceptance to its own terms, or the offeror objects within a reasonable time. The practical reason: in commercial transactions, purchase orders and confirmations almost never match word for word.
When Acceptance Takes Effect
When parties negotiate by mail, fax, or email, the exact moment of acceptance can be decisive. Under the mailbox rule, acceptance takes effect the moment the offeree sends it, not when the offeror receives it.5Legal Information Institute. Mailbox Rule Drop your signed acceptance in the mailbox on Monday, and if the offeror tries to revoke on Tuesday, you already have a contract. The letter can arrive Wednesday.
The mailbox rule is a default, not a mandate. Either party can specify in the offer that acceptance isn’t effective until received, and carefully drafted contracts often do. Option contracts also typically require the acceptance to reach the offeror before the deadline rather than simply be mailed by then.5Legal Information Institute. Mailbox Rule
The Consideration
Consideration is the reason each side enters the deal: the thing of value each party gives in exchange for the other’s promise. Without it, you have a gift or a bare promise, and courts won’t enforce either as a contract.6Legal Information Institute. Consideration Consideration doesn’t have to be money. It can be a promise to do something, a promise to refrain from doing something, goods, or services.7Legal Information Institute. Bargain What matters is that each party’s promise or action was given in exchange for the other’s.
Courts generally refuse to judge whether the consideration was a fair trade. You can sell a house for a dollar if both sides genuinely agree. The law cares that a bargain was struck, not whether it was a smart one. An extreme imbalance might prompt a closer look at fraud, coercion, or some other defect, but imbalance alone isn’t a reason to void a contract.6Legal Information Institute. Consideration
What Doesn’t Count as Consideration
Not everything that looks like a bargain qualifies. Three common traps:
- Past consideration. If someone already did something for you before any deal was discussed, that past act can’t serve as consideration for a new promise. A neighbor helps you move on Saturday, and on Monday you promise to pay them $200. The promise is unenforceable because the help wasn’t given in exchange for the money.
- Pre-existing duty. Doing something you were already legally required to do doesn’t count. If a contractor is already under contract to build your deck, promising to pay them extra for the same work doesn’t create a new enforceable agreement. If the contractor agrees to do genuinely additional work, that new effort can serve as fresh consideration.6Legal Information Institute. Consideration
- Illusory promises. A promise that leaves one party completely free to perform or walk away isn’t really a promise. If a contract says “we may provide services at our sole discretion,” that party hasn’t committed to anything. Both sides must be bound, or neither is.
When a Promise Without Consideration Still Binds
Sometimes a promise lacks consideration but still deserves enforcement. Promissory estoppel exists for those situations. When someone makes a clear promise, the other person reasonably relies on it, and that reliance causes real harm, a court can enforce the promise even without traditional consideration.8Legal Information Institute. Promissory Estoppel An employer who promises a job, prompting the candidate to quit their current one and relocate, may be held to that promise if they rescind the offer. Courts apply the doctrine cautiously, but it prevents a person from breaking a promise after someone else has already restructured their life around it.
What the Three Elements Don’t Cover
Having all three elements doesn’t guarantee a contract will stick. A few outside requirements run in the background.
Some agreements have to be in writing. Under the statute of frauds, real estate transactions, contracts that can’t be performed within a year, sales of goods worth $500 or more, and promises to pay another person’s debt generally need signed written evidence to be enforceable. The writing itself doesn’t have to be a formal contract. A signed letter, email, or text exchange can be enough if it identifies the parties, describes the essential terms, and is signed by the person being held to the deal.
Both parties also need legal capacity. Minors, generally anyone under 18, can enter contracts but can usually cancel them at will, with limited exceptions for necessities like food, medical care, and housing. A person who genuinely couldn’t understand the terms because of mental incapacity gets similar protection, and that contract is typically voidable.
The agreement has to be voluntary and honest. Duress, where improper threats push someone into signing, makes a contract voidable at the victim’s option. So does undue influence, where one party exploits a position of trust or power. Fraud and material misrepresentation have the same effect. Real agreement requires free will and honest dealing.
Mutual Assent Ties It Together
Offer and acceptance are really two halves of a larger idea called mutual assent: proof that both sides agreed to the same deal on the same terms.9Legal Information Institute. Mutual Assent Courts measure this objectively, looking at what the parties said and did rather than trying to read minds. If your words and conduct would lead a reasonable person to believe you agreed, you’re bound, even if you privately had reservations or misunderstood something you could have clarified.
That objective standard is why written contracts are so much safer than oral ones. When the terms sit on paper, there’s far less room for each side to remember the deal differently. Written or spoken, the structure is the same. One party made an offer with clear terms. The other accepted those terms without changing them. Both gave something of value in exchange. Pull out any piece, and the structure falls.