For example, res mancipi could only be transferred through the conveyance of mancipatio or in iure cessio.

             At the same time, the distinction between possession and ownership was fundamental to Roman law, seen as the difference between having a thing and being entitled to have it. Possession, as a fact, was protected in Roman law through possessory interdicts to refrain others from interfering with possession. "Possession is not merely holding, but holding in the manner of the owner"". Remedies for recovering possession, the requirement of possession for ownership and an understanding of possession as a 'holding in the manner of the owner' lead to a mixing of ownership and possession. .

             However, crucial differences remain. Possession required the physical manifestation of its existence as well as intent to possess, both of which were not required for ownership.

             Interdictal protection of possession had no reference to entitlement; it gave no indication of the protected possessor being the rightful owner. According to Gaius, interdicts only served to decide who would be the defendant in a proprietary action of vindicatio which would test the issue of entitlement. The possessor could claim remedy against his dispossessor, where the title of either party would be irrelevant to the proceedings. If the dispossessor were owner, he would have to bring in a separate action of vindicatio. Possession by itself would be protected from interference, however, combined with other factors, possession could become a method of acquiring ownership itself. .

             Acquisition of ownership (a right in rem) from a previous proprietor (derivative ownership) required a formal conveyance before ownership could be actually transferred. Mancipatio and in iure cessio were the required forms of conveyance for res mancipi and traditio (simple delivery based on a valid ground or just cause) was the required form of conveyance for other things.