Abstract: Community custom has played a limited but important role in the law of property. In addition to a few major historic examples such as mining camp rules and whaling, property law sometimes relies on community custom, for example in adverse possession, nuisance law, and beach access. This Article proposes an informational theory of custom in property law. Custom is subject to a communicative tradeoff in the law: all else being equal, informationally demanding customs require an audience with a high degree of common knowledge. General customs already known throughout society do not require much extra publicity from the law, and the law can easily draw on such customs. By contrast, customs that vary by community raise the question of the need for processing by non-expert audiences, i.e., outgroup dutyholders and government officials. This tradeoff helps explain the differential receptiveness to various customs and the process by which they are formalized if they are adopted into the law. The information-cost theory suggests that enthusiasts and skeptics of custom have both tended to ignore this process. The theory is then applied to some suggestive evidence from grazing customs and the pedis possessio doctrine in mining law, under which miners have pre-discovery rights to the spot being worked. Finally, the information-cost theory of custom sheds some light on the history and controversies over the numerus clausus (standardization and limitation of the set of basic property forms) and on the question of baselines of property entitlements in the law of takings.