Most, if not all, systems of law are compelled by the very nature of things to distinguish landed from all other types of property, for land is necessary for the production of food and the erection of dwelling-places, and, unlike other things, it cannot be moved. In English law, in the guise of the distinction between "real" and "personal" property, this contrast is particularly marked; in Roman law, though it existed as early as the XII Tables (1), its importance is overshadowed by a different and purely Roman distinction, that between res mancipi and res nec mancipi.
Res mancipi were land subject to Roman ownership (2), slaves, beasts of draft and burden, including cattle, and rustic servitudes belonging to land subject to Roman ownership; res nec mancipi were all other things. In the developed law the point of the distinction is that full Quiritarian ownership in res mancipi can only be transferred by the solemn method of conveyance known as mancipatio or the equivalent ceremony of in iure cessio, whereas the ownership of res nec mancipi can be transferred by mere delivery (traditio). Thus if A wants to make B the gift of a sheep and delivers it to him for that purpose the sheep becomes immediately the full Quiritarian property of B, but if he wants to give him an ox and does the same the ox remains the property ex iure Quiritium of A, because an ox is a res mancipi whereas a sheep is not.
