An obligation is contracted by question and answer, that is to say, by a form of words, when we stipulate that property shall be conveyed to us, or some other act be performed in our favour. Such verbal contracts ground two different action, namely condiction, when the stipulation is certain, and the action on stipulation, when it is uncertain; and the name is derived from stipulum, a word in use among the ancients to mean 'firm,' coming possibly from stipes, the trunk of a tree. 1 In this contract the following forms of words were formerly sanctioned by usage: 'Do you engage yourself to do so and so?' 'I do engage myself.' 'Do you promise?' 'I do promise.' 'Do you pledge your credit?' 'I pledge my credit.' 'Do you guarantee?' 'I guarantee.' 'Will you convey?' 'I will convey.' 'Will you do?' 'I will do.' Whether the stipulation is in Latin, or Greek, or any other language, is immaterial, provided the two parties understand one another, so that it is not necessary even that they should both speak in the same tongue, so long as the answer corresponds to the question, and thus two Greeks, for instance, may contract an obligation in Latin. But it was only in former times that the solemn forms referred to were in use: for subsequently, by the enactment of Leo's constitution, their employment was rendered unnecessary, and nothing was afterwards required except that the parties should understand each other, and agree to the same thing, the words in which such agreement was expressed being immaterial. 2 The terms of a stipulation may be absolute, or performance may either be postponed to some future time, or be made subject to a condition. An absolute stipulation may be exemplified by the following: 'Do you promise to give five aurei?' and here (if the promise be made) that sum may be instantly sued for. As an instance of stipulation in diem, as it is called where a future day is fixed for payment, we may take the following: 'Do you promise to give ten aurei on the first of March?' In such a stipulation as this, an immediate debt is created, but it cannot be sued upon until the arrival of the day fixed for payment: and even on that very day an action cannot be brought, because the debtor ought to have the whole of it allowed to him for payment; for otherwise, unless the whole day on which payment was promised is past, it cannot be certain that default has been made. 3 If the terms of your stipulation run 'Do you promise to pay me ten aurei a year so long as I live?' the obligation is deemed absolute, and the liability perpetual, for a debt cannot be owed till a certain time only; though if the promisee's heir sues for payment, he will be successfully met by the plea of contrary agreement.