Given the diversity and the heterogeneous nature of contractual relations, it is impossible to provide a uniform definition which would encompass all types of contracts and their specific features. For this reason, the legal literature includes various classifications of contracts, starting from the Roman law to the contemporary civil legislation. These classifications are based on different systematization criteria. In numerous classifications of contracts pertinent to both Roman and contemporary civil doctrine, the most prominent types of contracts are: a) unilateral and bilateral contracts; b) causal and abstract contracts; c) formal and informal contracts; d) onerous and gratuitous contracts; e) commutative (equivalent result) and aleatory (chance-dependant) contracts; f) principal and accessory contracts; g) personality and non-personality contracts; h) mortis causa and inter vivos contracts, and their combination; and i) fiduciary contracts. In the Roman law, contracts were initially classified on the basis of the (oral) form by which they were contracted; later on, when the written form prevailed, they were classified into contracts involving obligations resulting from some substantive act (performance) and contracts involving obligations resulting from mutual obligations of the contracting parties.