It will be noticed that cases (2) and (3) differ from (4) in an important respect. In (2) and (3) the contract is formed by one party doing all he can be required to do under the contract. The contract is formed by performance on one side, and it is this performance which makes obligatory the promise on the other. The outstanding obligation is all on one side. In (4) each party is bound to some act or forbearance in the future. There is an outstanding obligation on both sides. Where the benefit, in contemplation of which the promise is made, is done at the same time that the promise acquires a binding force, - where it is the doing of the act that concludes the contract, - then the act so done is called an executed or present consideration for the promise. Where a promise is given for a promise, each forming the consideration for the other, the consideration is said to be executory or future.
3 Great Northern R. Co. v. Witham, 9 Law Rep. (C. C. P.) 12, 19; Busher v. New York Life Ins. Co., 72 N. H. 551, 58 Atl. 41; Nicholson v. Acme Cement Plaster Co., 145 Mo. App. 523, 122 S. W. 773; 2 111. Law Review, 463 (Roscoe Pound). The word "unilateral" is also sometimes used to express the idea of an agreement not binding because lacking in mutuality of obligation. For criticism of this use, see High Wheel Auto Parts Co. v. Journal Co. of Troy, 50 Ind. App. 396 , 98 N. B. 442. See "Contracts," Dec. Dig. (Key-ilo.) § 10; Cent. Dig. §§ 21-40. Clark Cont.(3d Ed.) - 2