What is Accessio (Roman law)?

Accessio is a concept from Ancient Roman property law that decided ownership of property which is merged, or acceded to, another piece of property. In general the owner of the principal thing, whichever it is, became the owner of the acceded thing also. Its usage continues in modern times in legal systems around the world employing Roman property law, primarily civilian legal systems.

Accessio was not a specific rule of original acquisition of property in itself, instead it served as the principle underlying the modes of acquisition that had their own particular guidelines for determination of ownership.