An ejido is an area of communal land used for agriculture in which community members have usufruct rights rather than ownership rights to land, which in Mexico is held by the Mexican state. Peasants awarded ejidos in the modern era farm them individually in parcels and collectively maintain communal holdings with government oversight. Although the system of ejidos was based on an understanding of the preconquest Aztec calpulli and the medieval Spanish ejido, in the twentieth century ejidos are government controlled. After the Mexican Revolution, ejidos were created by the Mexican state to grant lands to peasant communities as a means to stem social unrest. The awarding of ejidos made peasants dependent on the government, with the creation of a bureaucracy to register and regulate them through the National Agrarian Registry. As Mexico prepared to enter the North American Free Trade Agreement in 1991, President Carlos Salinas de Gortari declared the end of awarding ejidos and allowed existing ejidos to be rented or sold, ending land reform in Mexico.