United Dominions Trust Ltd v Eagle Aircraft Services Ltd  1 WLR 74 Court of Appeal
Diplock LJ:… Under contracts which are only unilateral-which I have elsewhere described as ‘if’ contracts-one party, whom I will call ‘the promisor’, undertakes to do or to refrain from doing something on his part if another party, ‘the promisee’, does or refrains from doing something, but the promisee does not himself undertake to do or to refrain from doing that thing. The commonest contracts of this kind in English law are options for good consideration to buy or to sell or to grant or take a lease, competitions for prizes, and such contracts as that discussed in Carlill v Carbolic Smoke Ball Co  1 QB 256. A unilateral contract does not give rise to any immediate obligation on the part of either party to do or to refrain from doing anything except possibly an obligation on the part of the promisor to refrain from putting it out of his power to perform his undertaking in the future. This apart, a unilateral contract may never give rise to any obligation on the part of the promisor; it will only do so on the occurrence of the event specified in the contract, viz, the doing (or refraining from doing) by the promisee of a particular thing. It never gives rise, however, to any obligation on the promisee to bring about the event by doing or refraining from doing that particular thing. Indeed, a unilateral contract of itself never gives rise to any obligation on the promisee to do or to refrain from doing anything. In its simplest form (for example, ‘If you pay the entrance fee and win the race, I will pay you £100’), no obligations on the part of the promisee result from it at all. But in its more complex and more usual form, as in an option, the promisor’s undertaking may be to enter into a synallagmatic contract with the promisee on the occurrence of the event specified in the unilateral contract, and in that case the event so specified must be, or at least include, the communication by the promisee to the promisor of the promisee’s acceptance of his obligations under the synallagmatic contract. By entering into the subsequent synallagmatic contract on the occurrence of the specified event, the promisor discharges his obligation under the unilateral contract and accepts new obligations under the synallagmatic contract. Any obligations of the promisee arise, not out of the unilateral contract, but out of the subsequent synallagmatic contract into which he was not obliged to enter but has chosen to do so.