Libro Quinto · Título V Capítulo V. División de los contratos

Contracts are unilateral if the obligation falls solely upon one of the contracting parties; they are bilateral if both parties bind themselves reciprocally.

They are consensual when the consent of the parties suffices for them to be perfected; and real when delivery of the thing is required for their perfection.

They are principal, when they subsist by themselves; and accessory, when their object is the performance of another obligation.

An onerous contract is one in which reciprocal benefits and burdens are stipulated; and a gratuitous one, one in which the benefit is only to one of the parties.

The onerous contract is commutative when the performances owed by the parties are certain from the time the contract is concluded, in such a way that they can immediately assess the benefit or the loss that it causes them. It is aleatory when the performance due depends on an uncertain event that determines the gain or loss, from the moment such event occurs.

Contracts are conditional where their realization or their subsistence depends on an uncertain event or one unknown to the parties; and absolute, those whose realization is independent of any condition.