A valid contract has three elements: an offer, an acceptance, and consideration. Miss any one of them and a court will not enforce the agreement, no matter how sincerely the parties meant to be bound.1Legal Information Institute. Contract Two further requirements sit alongside the three — the parties must have legal capacity, and the purpose must be lawful — but offer, acceptance, and consideration are the building blocks, and the place most disputes begin.
Offer
A contract starts when one party proposes a deal with terms clear enough that the other party can simply say “yes.” The proposal has to reach the other person and spell out the important details, like what is being exchanged and at what price. “I’ll sell you my truck for $8,000” is an offer. “I might sell my truck someday” is not, because it lacks the specificity and intent that signal willingness to be bound.2Legal Information Institute. Offer
Revoking an Offer
An offeror can generally pull an offer off the table at any point before the other side accepts. The catch is that the revocation has to actually reach the offeree to take effect.3Legal Information Institute. Revocation Once the offeree communicates acceptance, revocation is too late. The timing matters more than people expect, especially with mailed or emailed responses where there is a gap between sending and receiving.
Are Advertisements Offers?
Usually not. Courts treat most store ads and online listings as invitations to negotiate rather than binding offers. The store is inviting you to come in and make an offer to buy, not promising to sell to everyone who walks through the door. The exception is when an ad pins down specific terms, like a limited quantity, an exact price, and a clear method for accepting. An ad stating “first 50 customers get a 65-inch TV for $200” is specific enough that a court could treat it as an offer.
Acceptance
Acceptance happens when the offeree agrees to the exact terms of the offer and communicates that agreement back. The agreement can be spoken, written, or shown through conduct, like shipping goods after receiving a purchase order.1Legal Information Institute. Contract What acceptance cannot be is silent. Failing to respond to an offer does not create a contract, even if the offeror says something like “I’ll assume you agree unless I hear otherwise.”
The Mirror Image Rule
Under the traditional common law approach, an acceptance has to match the offer exactly. Change even one term and you have made a counteroffer, which kills the original offer.4Legal Information Institute. Mirror Image Rule If someone offers to sell you 200 units at $15 each and you reply “I’ll take 200 at $12 each,” you haven’t accepted anything. You’ve rejected the offer and proposed a new one.
The Uniform Commercial Code softens this for sales of goods. Under the UCC, an acceptance that includes additional or different terms can still form a binding contract, as long as the response is clearly meant as acceptance rather than a fresh proposal.4Legal Information Institute. Mirror Image Rule
The Mailbox Rule
When parties aren’t communicating face to face, timing gets tricky. The mailbox rule says an acceptance takes effect the moment the offeree properly sends it, not when the offeror receives it.5Legal Information Institute. Mailbox Rule Drop a correctly addressed, properly stamped acceptance letter in the mailbox on Tuesday and the contract forms Tuesday, even if the letter arrives Friday. If the offeror mails a revocation on the same day you mail your acceptance, the acceptance wins, because it was effective on dispatch while the revocation only takes effect when you receive it.
Counteroffers
A counteroffer does double duty. It rejects the original offer and creates a new one in its place. The original offeror then becomes the offeree and can accept or reject the counteroffer.2Legal Information Institute. Offer People sometimes try to go back to the original terms after a counteroffer falls apart. That doesn’t work automatically. Once an offer is rejected through a counteroffer, it is dead unless the original offeror revives it.
Consideration
Consideration is what separates a binding contract from a bare promise. Each side must give up something, or commit to doing something, in exchange for what the other side provides. The formal description is a “bargained-for exchange.”6Legal Information Institute. Consideration Consideration doesn’t have to be money. It can be a service, a physical object, or a promise to stop doing something you have the legal right to do, like agreeing not to sue someone in exchange for a settlement payment.
Courts generally don’t care whether the exchange was a good deal. Selling a car worth $20,000 for $500 involves valid consideration on both sides, even though the values are wildly unequal. What matters is that each party bargained for and received something of recognized value.6Legal Information Institute. Consideration Extreme imbalance can raise suspicion of fraud or mistake, but a lopsided price alone doesn’t kill the contract.
Past Consideration Doesn’t Count
One of the most common traps involves past actions. If your neighbor mows your lawn without being asked, and you later promise to pay them $50 for doing it, the promise is generally unenforceable. The lawn was already mowed before you promised anything, so there’s no bargained-for exchange. The neighbor’s work wasn’t given in exchange for your promise. People get tripped up because the promise feels morally binding, but courts treat it as a gift promise with no legal teeth.
Promissory Estoppel: When Reliance Fills the Gap
Sometimes a promise lacks consideration but still deserves enforcement because someone relied on it to their detriment. The doctrine of promissory estoppel fills that gap. If one party makes a clear promise, the other party reasonably relies on it, and walking away would cause real harm, a court can enforce the promise even without traditional consideration.7Legal Information Institute. Promissory Estoppel An employer who promises a job, prompting the candidate to quit their current position and relocate, may be held to that promise even without a formal employment contract. Courts apply the doctrine sparingly, and typically only when the injustice is clear.
The Two Requirements That Sit Alongside the Three
Offer, acceptance, and consideration can all be present and the agreement still won’t hold up if either party lacked legal capacity or the purpose was unlawful.1Legal Information Institute. Contract
Capacity
Both parties need the legal ability to enter a contract. That generally means being at least 18 and of sound mind. A contract signed by someone who lacks capacity is typically voidable, meaning the person without capacity can walk away or let the deal stand.8Legal Information Institute. Capacity A minor who buys a laptop can return it and get their money back, but the adult seller cannot use the minor’s age as an excuse to cancel. The power to undo the contract belongs only to the party who lacked capacity. The main exception involves necessities like food, clothing, shelter, and medical care, where a minor generally owes the reasonable value of what was provided.
Legality
A contract has to serve a lawful purpose. An agreement to split profits from an illegal gambling operation or to pay someone for committing a crime won’t be enforced, no matter how precisely drafted.1Legal Information Institute. Contract Courts will not compel performance of an illegal contract or award damages for its breach.
When a Contract Has to Be in Writing
Oral contracts are generally enforceable. A verbal agreement to buy a used couch for $200 is just as binding as a written one. Certain categories, though, must be in writing under a longstanding rule called the Statute of Frauds. The common ones are real estate transactions, sales of goods for $500 or more under the UCC,9Legal Information Institute. UCC 2-201 – Formal Requirements Statute of Frauds contracts that cannot be completed within one year, promises to pay someone else’s debt, and contracts made in consideration of marriage, like prenuptial agreements. The writing doesn’t have to be a formal document. A signed letter, email, or text message chain can satisfy the requirement if it identifies the parties, describes the subject matter, and is signed by the person being held to the deal.
What Can Undo a Validly Formed Contract
Even when all elements are present and any writing requirement is met, certain circumstances can make a contract voidable after the fact.
Duress
A contract signed under threats or coercion is voidable by the pressured party. Duress means one side used unlawful threats or behavior that destroyed the other party’s ability to exercise free will.10Legal Information Institute. Duress The classic example is signing at gunpoint, but duress can also involve economic threats, like a vendor threatening to breach a critical contract during a crisis unless the other side agrees to extortionate new terms.
Fraud and Misrepresentation
If one party lies about something important to get the other side to agree, the deceived party can void the contract. The misrepresentation has to be material, meaning it’s the kind of false statement that would influence a reasonable person’s decision to enter the deal. Selling a car while hiding known engine damage is a textbook example. The deceived buyer can rescind the agreement and potentially recover damages.
Undue Influence
Undue influence is subtler. It occurs when someone in a position of trust or authority exploits that relationship to pressure another person into a contract. A caretaker persuading an elderly person to sign over property, or an attorney steering a client into an agreement that primarily benefits the attorney, are the kinds of situations courts scrutinize. The contract is voidable because the weaker party’s consent wasn’t truly free.
Putting the Elements Together
A clear offer, an unqualified acceptance, and real consideration on both sides form a contract. Add two parties with legal capacity pursuing a lawful purpose, and the agreement becomes enforceable.1Legal Information Institute. Contract Remove any single element and a court won’t enforce the deal or award damages for a breach. The absence of one piece doesn’t just weaken a contract. It prevents one from existing at all.