The idea that private ownership implies that owners are free to do with their things whatever they want is shown to be mistaken. It is argued that private, public and common ownership are all based on the right to alienate a thing, regardless of the number of owners. Social or legal norms can make the ownership of a thing conditional on the participation in government or on group membership. In the former case, the norms establish public ownership, in the latter case common ownership. If things are owned and these norms do not apply, they are privately owned. Local social circumstances determine to some extent what form of ownership generates the highest benefits to owners, social and legal norms provide incentives which encourage or discourage the choice of the efficient form of ownership.