This chapter addresses the Roman law of ownership and the rights that modified it, including, for instance, the rights of predial servitude and usufruct. Classic Roman jurists focused on private property over other kinds, such as sacred property and public property. The largest division of Gaius's classification preserved by Justinian in his Institutes, concerned the law dealing with things. According to their physical nature or legal conditions, things could be classified in many different ways. The main form of ownership was that recognized by Roman civil law. Co-ownership implied ownership of the same thing by two or more owners at the same time. The most important distinction in the law of property was that between ownership and possession. Because "ownership has nothing in common with possession", interdictal protection could be used even against owners who had lost possession justly but then recovered it unjustly. The usufruct was basically constituted through a legacy.