Contract law deals with promises which create legal rights. In most legal systems, a contract is formed when one party makes an offer that is accepted by the other party. Some legal systems require more, for example that the parties give each other, or promise to give each other, something of value. In common law systems, a one sided promise to do something (e.g. a promise to make a gift) does not lead to the formation of an enforceable contract, as it lacks consideration.