In the early Middle Ages, improved land was scarce and unimproved land abundant. As a result, lords and villages chose different property rights to govern these two types of land. Property rights to improved land were precisely assigned, giving rise to what we call private property. In contrast, it was not worthwhile to define property rights to unimproved land clearly, for enforcing such rights would have required monitoring unwarranted by the low value of the land. Thus, unimproved land fell under a regime of common rights. This system was efficient as long as it was possible to improve land when relative prices shifted. Yet poorly defined property rights would come to stand in the way of improvements in general and water control in particular. In order to earn a return on their land improvement schemes, water control projectors had to divide costs and benefits between themselves and the owners of the land. Carrying out such a division required an agreement on existing property rights (establishing who owned the marsh) as well as agreement on a rule that translated the original distribution of property rights into a share of the profits (benefits minus costs). Presumably, marshes could have been improved and still remain common. However, in the eighteenth and nineteenth centuries it was generally thought that development was incompatible with common rights. To complicate matters, by the year 1700, altering property rights from common to private required state approval. Hence, the regulation of common land that had begun largely as private contracts between medieval lords and their serfs had become a public institution.