Ask what is exchanged to create the obligation. In a bilateral contract, one promise is exchanged for another promise. A buyer promises to pay, and a seller promises to convey, on the agreed terms. In a unilateral contract, the offer asks for an act in exchange for a promise. Think of a reward offered for returning a lost item. The requested performance is the key to acceptance. Do not count the number of people or signatures to decide the category. Compare promise for promise with promise for performance. Rules about beginning performance and revocation can depend on the circumstances and governing law. Quick check: buyer and seller exchange promises to buy and sell. Which pattern is that? Bilateral. Each side makes a promise.