Offer and acceptance

Offer and acceptance is the first doctrinal gateway into Contract Law. It answers a deceptively simple question: when have parties crossed the line from negotiation into legal commitment? The answer matters because English law does not enforce every promise, commercial assurance, expression of interest, or moral undertaking. It enforces bargains which satisfy the requirements of formation: agreement, consideration, intention to create legal relations, and sufficient certainty. Week 1 is concerned with the agreement element.

The classical model presents agreement as a sequence: one party makes an offer; the other accepts it; a contract is thereby formed. An offer is an expression of willingness to contract on specified terms, made with the intention that it shall become binding as soon as it is accepted. Acceptance is an unqualified assent to those terms, communicated in the required manner, unless an exception applies. This model is powerful because it supplies a clear structure for problem questions. It asks: what was the offer; was it still open; was the response an acceptance, counter-offer, request for information, or rejection; was acceptance communicated; and what terms were incorporated at that moment?

The model is also limited. Many modern contracts are not made by a neat exchange of offer and acceptance. Construction projects, supply contracts, online platforms, battle-of-forms disputes and long negotiations often proceed through conduct, draft terms, purchase orders, delivery, performance, and payment. Courts therefore insist on an objective approach. The question is not what either party secretly intended. It is what a reasonable person, knowing the relevant background available to both parties, would conclude from their words and conduct. The law is concerned with external manifestation of assent.

For Durham first-year purposes, this topic is foundational in two respects. First, it supplies the technical vocabulary used throughout Contract Law: offer, acceptance, counter-offer, revocation, communication, certainty and objective intention. Secondly, it introduces the method expected in first-year legal analysis. You must move carefully between rule, authority, fact and policy. Do not treat cases as isolated stories. Boots explains why shop displays are normally invitations to treat; Carlill shows that an advertisement may exceptionally be an offer; Hyde v Wrench demonstrates the destructive effect of a counter-offer; Entores and Brinkibon show how communication rules adapt to technology; RTS demonstrates the modern willingness to find a contract from performance despite incomplete paperwork.

The crucial discipline is chronological analysis. Formation occurs, if at all, at a particular moment. Once that moment is identified, later regret, later silence, or later attempts to improve the bargain do not normally alter the existence of the contract. In problem questions, draw a timeline before writing. In essays, contrast the apparent certainty of the classical model with the judicial pragmatism required by modern commercial practice.

The language of offer and acceptance is a product of nineteenth-century contract theory. Earlier English law did not organise contractual liability around a single analytical model of agreement. Actions in debt, covenant and assumpsit developed through forms of action, procedural categories and evidential concerns. The modern law emerged as contract became a generalised body of doctrine suited to an expanding market economy. Judges and treatise writers increasingly described contractual obligation as founded upon consent, bargain and exchange.

Nineteenth-century lawyers were influenced by will theory: the idea that contractual obligation derives from the parties' voluntary choice to bind themselves. Offer and acceptance appeared to give legal form to that idea. One party proposes terms; the other assents; mutual consent produces obligation. This was attractive because it seemed principled, liberal and commercially useful. It respected autonomy while allowing courts to identify a definite moment at which rights and duties arose.

Yet English law never adopted a purely subjective theory of will. The common law needed reliable rules for commerce. Parties often disagree after the event about what they intended. If private intention governed, contractual liability would be unstable. The law therefore moved toward objectivity: a person is bound by the reasonable meaning of their outward conduct, not by undisclosed mental reservations. This explains why offer and acceptance is not an exercise in psychological reconstruction. It is an inquiry into communications, documents, conduct and context.

The nineteenth-century cases also established formal distinctions that remain central. In Hyde v Wrench, the Court of Chancery held that a counter-offer rejects the original offer. In Brogden v Metropolitan Railway, the House of Lords recognised that conduct may evidence acceptance. In Carlill, the Court of Appeal held that a public advertisement could be a unilateral offer where its language and surrounding circumstances showed an intention to be bound. These cases remain prominent because they supply doctrinal categories still used in undergraduate problem questions.

The twentieth century complicated the model. Mass retailing required courts to decide whether displays of goods were offers or invitations to treat. Pharmaceutical Society of Great Britain v Boots Cash Chemists treated goods on shelves as invitations to treat, preserving the retailer's power to accept or reject the customer's offer at the till. The rise of instantaneous communications required courts to distinguish postal acceptance from telex, fax and later electronic communication. Entores and Brinkibon refused to extend the postal rule to instantaneous messages, preferring a receipt-based analysis tempered by practical judgement.

Late twentieth- and early twenty-first-century contracting further reduced the adequacy of the classical model. Commercial parties frequently begin performance before signing the final contract. Standard terms are exchanged in competing forms. Letters of intent are used to keep projects moving. Courts then ask whether, objectively, the parties intended to be bound and whether the essential terms are sufficiently certain. RTS Flexible Systems v Müller is the leading modern statement: the absence of a signed final document is not conclusive if conduct demonstrates agreement.

The historical lesson is therefore twofold. The offer-and-acceptance model is not obsolete. It remains the ordinary analytical starting point and is indispensable in first-year problem solving. But it is a model, not a metaphysical truth. English law uses it when it illuminates agreement and relaxes it when insistence on form would defeat the reasonable meaning of the parties' conduct. That tension between certainty and realism is one of the main themes of Contract Law.

1. Agreement is judged objectively. The court asks what a reasonable person would understand from the parties' words and conduct. Secret intention is irrelevant unless communicated. This is why a joking advertisement, a careless email, or an informal exchange may have legal consequences if objectively promissory. Conversely, elaborate negotiations may fail to bind if they remain expressly subject to contract or leave essential matters unresolved.

1. An offer must be distinguished from an invitation to treat. An offer is capable of immediate acceptance. An invitation to treat is a preliminary step inviting offers. Shop displays, catalogues and most advertisements are normally invitations to treat. The policy is practical: traders must retain control over stock, pricing errors and suitability of customers. But the classification depends on objective intention. Carlill shows that an advertisement can be an offer where it is specific, promissory and indicates seriousness, such as by referring to money deposited to show sincerity.

1. Bilateral and unilateral contracts differ in structure. In a bilateral contract, each party undertakes a promise to the other: I promise to sell; you promise to buy. Acceptance is usually a communicated promise. In a unilateral contract, the offeror promises to pay or confer a benefit if the offeree performs an act: find my dog and I will pay £100. Acceptance normally occurs by performance of the stipulated act. Carlill is the standard example. The distinction matters because unilateral offers raise special questions about communication, revocation and whether the offeror may withdraw once performance has begun.

1. Acceptance must correspond with the offer. The mirror image rule requires acceptance to be unqualified. A purported acceptance which changes the terms is a counter-offer, not an acceptance. Hyde v Wrench is the classical authority. A counter-offer destroys the original offer unless the offeror renews it. By contrast, a mere request for information does not reject the offer. The difference is often tested in problem questions. Asking whether payment may be made in instalments may be inquiry; saying I accept if payment may be made in instalments is conditional acceptance and therefore not acceptance.

The leading cases form a coherent map of formation doctrine. They should not be memorised as disconnected authorities. Each case answers a recurring question.

Carlill v Carbolic Smoke Ball Co is the classic authority for unilateral offers to the world. The company advertised that it would pay £100 to anyone who used its product as directed and still contracted influenza. The Court of Appeal treated the advertisement not as mere puff but as an offer capable of acceptance by performance. The deposit of money in a bank was evidence of seriousness. The case is important because it prevents a rigid rule that advertisements are always invitations to treat. Classification depends on objective intention.

Pharmaceutical Society of Great Britain v Boots Cash Chemists concerns self-service shopping. The display of goods on shelves was held to be an invitation to treat; the customer made the offer by presenting goods at the till; acceptance occurred when the cashier completed the sale. This preserves the shopkeeper's freedom to refuse sale and avoids impossible consequences if every display were an offer to sell to anyone who picks up the goods.

Hyde v Wrench supplies the counter-offer rule. The seller offered to sell a farm for £1,000. The buyer replied with £950. When the buyer later tried to accept the original £1,000 offer, it was too late: the counter-offer had rejected and extinguished the original offer. The rule is formal, but its practical function is certainty. Parties must know whether an offer remains open.

Felthouse v Bindley is the orthodox authority that silence is not acceptance. The uncle purported to buy his nephew's horse and said that if he heard no more he would consider the horse his. The nephew subjectively intended to sell, but had not communicated acceptance. The case illustrates the objective approach and protects offerees from being forced to respond to unwanted offers.

Entores v Miles Far East Corporation and Brinkibon address instantaneous communications. Entores held that telex acceptance takes effect when received, not when sent. Denning LJ's examples show why: if the line goes dead, or the message is not received, the offeree must know that no contract has yet been made. Brinkibon later warned that no universal rule can resolve every case involving modern communications. The court must consider intention, business practice and risk.

Butler Machine Tool v Ex-Cell-O is the canonical battle-of-forms case. Each party attempted to contract on its own standard terms. The Court of Appeal rejected a simplistic assumption that both sets of terms could coexist. The case is often associated with the last-shot approach, though Lord Denning MR preferred a more flexible analysis. It remains essential for problem questions involving purchase orders, acknowledgements and delivery notes.

RTS Flexible Systems v Müller is the modern corrective to excessive formalism. The parties negotiated detailed terms, began performance and did not execute the final contract in the manner contemplated. The Supreme Court held that a contract had nevertheless arisen. The case is a reminder that English law does not worship paperwork at the expense of objectively manifested agreement. If parties act as though bound, courts may find a contract, provided the essential terms are sufficiently certain.

See practice questions section below.

See further reading section below.