Agreement originates, as will be discussed in detail hereafter1 in offer and acceptance. Since a contract involves the idea of an outstanding and enforceable promise we must consider briefly the methods in which such an agreement may come into existence. (1) A may make a promise and B may accept it. If A's promise does not require B to do or forbear any right we have here a promise, but no simple contract, since no consideration exists.2 Where the seal retains its common-law effect, a contract of this sort may be made under seal. (2) A may make a promise conditioned on B's making a promise3

1 See ch. V.

2 See ch. XIX.

3 See ch. XXXIX.

in return. B may make such promise. We have here a contract. Bach party has an outstanding executory promise enforceable against himself.4 (3) A may make a promise conditioned on B's doing a specified act. B can not accept this promise in any way other than by doing such act. If he does such act there is no outstanding promise enforceable against B. The only liability existing is against A. This, however, comes within the modern idea of a contract. (4) A may tender an act to be done if B will make a promise. If B accepts by making such promise, there is no outstanding executory liability enforeable against A. The only execu-tory promise is that of B. This, too, comes within the modern idea of a contract. (5) A may tender an act to be performed if B performs an act in return. B can not accept in any way except by performing such act. If he does so accept, no outstanding executory promise exists against either party. This does not fall within the modern idea of a contract.