Derivative Acquisition (Roman)

Derivative Acquisition (in Roman Law) covers the transfer of ownership from one person. The opposite being Original Acquisition. It is by far the more common way to obtain ownership of an item.

A contract itself does not transfer possession in Roman Law, instead the item passes by agreement. The Contract allows the new owner to retain detention after the transfer.

Mancipatio
Mancipatio is a formal transfer of ownership. In addition to the acquirer and seller there must also be 5 witnesses and a person with scales to formalise the transfer.

For some items, it is the only valid form of sale. These include slaves, beasts of burden, land within Italy, some servitudes, and incorporeal things.

Traditio
Traditio is an informal transfer of ownership, it just requires possession to be transferred (delivery), and some "casuse" that makes the parties intention clear.

Delivery can be "long handed" (Traditio longa manu), short handed (traditio brevi manu) or take the form of Constitutum possessorium.

In Justinian's age this was joined by Taditio Incertae Personae, delivery to an unknown person (Institutes 2.1.46). This allowed possession and ownership to pass when a specific person was not in mind; only the lack of knowledge of the recipient's identity varied this from other forms of delivery.