In all postfeudal legal systems, the basic ways of owning property are limited in number and standardized, in the sense that courts will enforce as property only interests that are built from a list of recognized forms. In the common law, this principle has no name and is invoked only semiconsciously; it is known in the civil-law tradition as the numerus clausus. This Article adopts this term for the corresponding understanding in the common law and advances an information-cost theory that explains the ubiquity and durability of the doctrine.

The numerus clausus can be seen at work in a variety of areas, including estates in land, concurrent interests, nonpossessory interests, interests in personal property, and intellectual property. This Article argues that the principle serves to reduce third-party information costs throughout the law of property. Because of their in rem nature, property rights give rise to third-party information costs in a way that contract rights do not. Individuals trying to avoid violating property rights or investigating whether to acquire them need to gather information. Those creating property rights will in some situations have too little incentive to conform to the most popular forms, requiring a degree of mandatory standardization. As it operates in practice, the numerus clausus strikes a rough balance between the costs of frustrating parties' objectives on the one hand and the costs of complicating third-party information-gathering on the other.

This Article demonstrates that this information-cost theory provides a better account for the numerus clausus than do alternative positive and normative views, including those based on network effects, sufficiency of notice, private standards, antifragmentation concerns, and the increasing importance of contract-based rights. Finally, this Article shows that, because it tends to preclude judicial innovation in the basic forms of property rights, the numerus clausus acts as an institutional-choice mechanism that channels to legislatures the power of innovation in the realm of property rights. In keeping with the basic information-cost theory, legislative creation and abolition of property rights is likely to lead to lower information costs than would judicial entrepreneurship in this area.

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