Types of contracts

Gaius (3.89) offered a fourfold classification of contracts, which Justinian followed (Inst. 13.3.2). According to Gaius, contracts were concluded by the delivery of a thing (re), by the uttering of formal words (verbis), by a doc­umentary form (litteris), or by formless consent (consensu).

The distinction between unilateral and bilateral contracts was also important. Unilateral contracts were those that created duties for one of the parties and, as a result, rights to the other (e.g., a stipulation or loan of money). The old Roman law was founded on unilateral obligations, probably because they constituted the most elementary type of obligations. Bilateral contracts were those that created reciprocal obligations, producing both rights and duties for each party (e.g., sale and partnership). The purpose of the bilateral obligation could not be fully achieved by means of two different interconnected unilateral contracts. For this reason, bilateral contracts constituted a masterpiece of late Republican jurisprudence.

Bilateral contracts were called imperfect (e.g., a mandate or deposit) when the obligations of one party were only contingent - for instance, in case of expenses or damages. Reciprocity was called perfect (e.g., a sale, lease, or partnership) when the obligations of both parties were inevitable. Thus, in a perfect contract it was not possible to have a sale in which the buyer had no obligation to pay or the seller no obligation to deliver, while in an imperfect contract it was possible to have a contract of deposit in which no obligation bound the depositor: e.g., if Titius gave a ring to Caius to care for but could demand its return with no expenses or damages.