Contracts involve a legal exchange of promises to complete an action, meet terms or complete an agreement. Parties breach a contract when the person fails to perform the duties assigned by the agreement, but death makes the performance of the duties impossible. In a unilateral, or one-sided, contract, one party, known as the offeror, makes a promise in exchange for an act (or abstention from acting) by another party, known as the offeree. If the offeree acts on the offeror’s promise, the offeror is legally obligated to fulfill the contract, but an offeree cannot be forced to act (or not act), because no return promise has been made to the offeror. Exchange of contracts is when the two legal firms representing the buyer and seller swap signed contracts, and the buyer pays a deposit. At this point, an agreement to buy or sell a property becomes legally binding: once everyone in the chain has exchanged, no one can back out of the deal.