What Makes a Contract Enforceable: A Common-Law Primer

A contract does not exist because a document says "Contract" at the top. It exists because the parties did specific things that the law treats as creating enforceable obligations. This matters because people sign what look like contracts every day that aren't enforceable, and make informal agreements every day that are. Knowing the difference is the first step to reading any contract — and to understanding when the absence of a signed document is not actually the absence of a deal.

This article covers the five classical elements of contract formation as they apply across the common-law tradition — the US, the UK, Canada, Australia, Ireland, and New Zealand all share a broadly common framework, inherited from English law and refined by each jurisdiction's case law. It is a factual overview, not legal advice. Cornell's Legal Information Institute entries on contract, offer, and consideration are useful starting references, as is the UK Consumer Rights Act 2015 where consumer contracts are involved.[¹][²][³][⁴]

The five elements

A valid and enforceable contract in common-law jurisdictions generally requires:

Offer — one party proposing definite terms.
Acceptance — the other party agreeing to those terms.
Consideration — something of value exchanged (or a valid substitute like a deed).
Intention to create legal relations — both parties intending legal consequences.
Capacity — both parties legally able to contract.

If any element is missing, the purported contract is typically not enforceable as such, although related doctrines (estoppel, quantum meruit, restitution) sometimes provide remedies anyway.

Offer

An offer is a definite proposal to be bound on specific terms, made in a way that makes clear the offeror is ready to contract if the offeree accepts. Three practical distinctions matter:

Offer versus invitation to treat. An advertised price in a shop window is typically not an offer — it's an invitation for the customer to offer to buy at that price, which the shop can accept or refuse. The same applies to goods on a shelf, items in a catalogue, and most price lists. When the buyer brings goods to the till, they make the offer; the cashier accepts by ringing up the sale.[²]
Offer versus statement of intention. "I'm thinking of selling my car for £5,000" is not an offer. "I'll sell you my car for £5,000" is. The distinction is whether the statement is made with the intention that acceptance would create a binding contract.
Offer versus invitation to tender. A request for tenders is usually an invitation to treat, meaning the tendering parties make offers and the requesting party chooses which to accept. Specific tender processes can flip the relationship (binding pre-contract obligations to consider tenders fairly, for example), but the default is an invitation to treat.

An offer must be definite. "I'll sell you my car for a fair price" is too vague. "I'll sell you my 2020 Toyota Corolla, registration number ABC123, for £8,500, delivery next Tuesday" is definite enough to accept.

An offer lapses:

If the offeree rejects it.
If the offeree proposes different terms (a counter-offer is legally a rejection followed by a fresh offer).
After a reasonable time if no time is specified.
At any time before acceptance if the offeror revokes it (subject to narrow exceptions for options supported by consideration, firm offers under the UCC in US sales of goods, and similar doctrines).
Automatically on the offeror's death in most circumstances.

Acceptance

Acceptance is the offeree's unconditional agreement to the exact terms of the offer. The core rule is the "mirror image" rule: acceptance must match the offer. A response that changes any term is a counter-offer, not an acceptance.

Three acceptance doctrines come up often:

Postal rule. A long-standing English (and widely adopted common-law) rule that acceptance by post is effective when the letter is posted, not when it is received. The rule is narrow in modern practice — courts usually hold that email and other instantaneous communications are subject to the standard "receipt" rule — but the postal rule still applies to some written acceptances.
Acceptance by conduct. A party can accept by doing what the offer asks, without formal words of acceptance. Performing a contract after receiving signed terms is typically acceptance of those terms; paying an invoice is typically acceptance of the underlying deal.
"Battle of the forms." When parties exchange standard terms (buyer's purchase order followed by supplier's terms of sale), the common-law "last shot" rule holds that the last set of terms sent before performance prevails. UCC §2-207 in the United States modifies this substantially for sales of goods; the UK's traditional approach is closer to the classical last-shot rule but with exceptions.

Silence is not acceptance. An offer that says "if you don't reply within 7 days you will be deemed to have accepted" does not, by itself, bind the offeree. There is no obligation on the offeree to act.

Consideration

Consideration is something of legal value exchanged between the parties — a promise, an act, a payment, a forbearance from action. It is the doctrine that distinguishes a contract from a gratuitous promise.[³]

The rules are old but still apply:

Consideration must move from the promisee. In classical doctrine, only the party giving consideration can enforce the promise. Many jurisdictions have narrowed this rule considerably (the UK Contracts (Rights of Third Parties) Act 1999, for example), but the underlying principle still matters.
Consideration need not be adequate, but must be sufficient. Courts do not police whether consideration is proportionate to the promise. Selling a house for £1 is, in principle, a valid contract — the consideration is sufficient because it is something of legal value, even if inadequate in value terms.
Past consideration is no consideration. A promise made after the fact to reward something already done is generally not enforceable as a contract. If A fixes B's car as a favour and B later promises to pay for it, B's promise is not enforceable as a contract because the act was already complete when the promise was made.
Pre-existing duty is not consideration. A promise to do what you are already legally obliged to do is not consideration for a new promise. Important for renegotiation of contracts mid-performance.

Alternatives to consideration. Some common-law jurisdictions enforce serious promises made by deed (a formal document, usually signed, witnessed, and delivered) even without consideration. Promissory estoppel — where one party reasonably relies on another's promise to their detriment — can also make a promise enforceable in certain circumstances, although typically as a shield rather than a sword.

Intention to create legal relations

Even when offer, acceptance, and consideration all exist, a contract is only enforceable if the parties intended legal consequences. Courts apply a presumption that depends on context:

Commercial context. Strong presumption that the parties intended legal relations. A written commercial deal, a signed purchase order, a services agreement — all presumed contractual. The presumption can be rebutted by express words ("This letter is non-binding and does not create legal relations") or by context that indicates the parties did not intend a contract.
Social and family context. Opposite presumption. Promises between family members, social arrangements, and casual agreements are presumed NOT to create legal relations. The presumption can be rebutted by clear evidence that the parties did intend legal consequences — a written agreement between a parent and adult child setting out financial terms, for example.
Comfort letters and letters of intent. Often intentionally drafted to sit between "contract" and "mere statement of intention". The specific language matters — "binding" versus "subject to contract" versus "intended to be morally binding only" each has a different legal effect.

Capacity

Both parties must be legally capable of contracting. The main categories of capacity issues:

Minors. Contracts with minors are typically voidable at the minor's option, with exceptions for necessaries (food, clothing, education) and specific statutory regimes. Practical implication: businesses usually avoid contracting directly with minors.
Mental capacity. A party who lacks the mental capacity to understand the contract at the time of signing may be able to avoid it, particularly if the other party knew or should have known of the incapacity. Intoxication is treated similarly when it is severe enough to prevent comprehension.
Corporate capacity. Companies contract through agents (directors, officers, authorised employees). The authority of the agent matters — a deal signed by someone without authority may not bind the company, although the company may be bound by apparent authority or by subsequent ratification.
Illegality. A contract for an illegal purpose is typically void. The contract cannot be enforced regardless of whether the other four elements are present.

When an otherwise valid contract still fails

Even with all five elements, a contract can fail for other reasons:

Duress and undue influence. A contract signed under threat or under improper influence may be voidable.
Misrepresentation. A contract induced by a false statement of fact may be voidable, and in some cases the victim can claim damages in addition.
Mistake. A serious mutual mistake about a fundamental term can void a contract. Unilateral mistakes are more limited.
Unconscionability. An exploitative contract between parties of grossly unequal bargaining power may be unenforceable in whole or in part.
Statutory unfairness regimes. In consumer contracts (UK Consumer Rights Act 2015, Australian Consumer Law, similar regimes elsewhere), unfair terms are not binding on the consumer regardless of formal contract validity.[⁴]

Practical implications

Three practical takeaways for anyone reading a document:

The name of the document matters less than its contents. A document labelled "Memorandum of Understanding" can be a contract if the five elements are present; a document labelled "Contract" may not be one if they are not. For the MOU question specifically, see our article on contract versus memorandum of understanding.
Informal agreements can still be contracts. An email exchange, a text message, a verbal agreement with a written follow-up — each can constitute a binding contract if the elements are present. The absence of a formal signed document is not proof of no contract.
A signed document with missing elements can still produce obligations. Promissory estoppel, quantum meruit, restitution, and other equitable doctrines can produce outcomes similar to contract enforcement when the formal contract fails but one party has relied on the other's promise.

Contract formation is the first filter. Everything else — reading specific clauses, assessing fairness, negotiating revisions — follows from it. The clauses in a contract only matter if the contract itself is enforceable, and the contract is only enforceable if the five elements above are present.