This Article examines a characteristic of property entitlements fundamental to the structure of property systems that has received scant academic attention, a characteristic referred to as the mutual exclusivity principle. According to this principle, a property system does not allow for the existence of incompatible rights. Two people cannot separately be the owners of the same resource, for instance. By contrast, two people can each hold valid but contradictory contract rights to the resource. Although the existing property literature has stressed the “exclusive” nature of property, the various ways in which property is imagined to be exclusive, such as by conferring “rights to exclude,” fail to capture the essence of property as a distinct legal institution. Unlike these alternative conceptions of exclusiveness, the mutual exclusivity principle holds true across the range of different types of property entitlements, including not just fee simple ownership but also security interests and servitudes, and across the range of assets subject to property law, including not just land and physical objects but also intangibles like intellectual property and corporate shares. Recognizing the role of the mutual exclusivity principle yields a number of practical insights. It helps explain various institutional features of property law, such as the system of future interests, the use of possession-based rules, the role of recording systems, and the negative, thing-based structure of property entitlements. It illuminates connections between property and other fields like corporations law and it calls into question aspects of existing doctrine, such as the preferred status of exclusion rights under the U.S. Constitution’s Takings Clause. It also modifies the influential theory that property law is heavily shaped by problems of high information costs: while existing accounts seem to suppose that property law entails relatively high information costs because it imposes a relatively broad set of duties on others, many of the information cost problems identified in the literature actually result from the mutual exclusivity problem, rather than from the breadth of property duties. At a more general level, understanding the centrality of the mutual exclusivity principle suggests some change in direction is called for within the wider property literature. American property scholarship has been preoccupied with questions about the scope and strength of property rights, overlooking the separate problem of ascertaining who happens to hold a given right, a problem distinctive to property law. Property, this Article argues, is at least as much about title chains, patent searches, and creditor priorities as it is about trespass, remedies, and eminent domain. [ABSTRACT FROM AUTHOR]