The "Reconquista" was a process not only of war and conquest, but also of repopulation. Christian kings moved their own people to locations abandoned by Muslims in order to have a population capable of defending the borders. The main repopulation areas were the Douro Basin (the northern plateau), the high Ebro valley (La Rioja) and central Catalonia. The repopulation of the Douro Basin took place in two distinct phases. North of the river, between the 9th and 10th centuries, the "pressure" (or "presura") system was employed. South of the Douro, in the 10th and 11th centuries, the "presura" led to the "charters" ("forais" or "fueros"). "Fueros" were used even south of the Central Range.