The rule that an acceptance is effective when posted when that method of response is appropriate. The exchange of commitments between parties to a contract often stated as the price one is willing to pay for the promise of another. A necessary ingredient for a contract. A one-sided promise given without any reciprocating promise in return and is not binding on the promisor. A real and substantial interest in specific property. The parties should be serious and expect that legal consequences should flow from their agreement. A necessary ingredient for a contract. As much as is deserved. Reasonable price paid for requested services where there is no actual contractual obligation