This system of immunity was greatly extended by the Carolingian sovereigns, but with two important changes. (1) Henceforward the privilege was seldom granted to laymen, but was bestowed as a matter of course on the estates of bishops and of religious houses. (2) The holders of such ecclesiastical estates were compelled to vest their powers of police and justice in the hands of laymen (_advocati_) chosen either by the central power or by some approved form of election. The intention of these changes was to use the private courts for the maintenance of public order, to extract the sting from a dangerous privilege, and to make it a serviceable instrument of royal policy. But only one half of the scheme was permanent. By the middle of the ninth century, when _immunitas_ had been granted to all religious foundations, the Carolingians allowed the right of choosing the _advocati_ to slip from their feeble grasp. The privileged estates remained, but the royal control over their internal government was gone. They became ecclesiastical seignories; whatever checks were imposed upon the power of their rulers came from the lay-nobles who were their neighbours, or from the subject population. Partly from respect for custom and tradition, partly from motives of self-interest, the great ecclesiastical landowners sided with the Crown, even in the tenth century, when the fortunes of royalty were at their lowest ebb. But for this support a price had to be paid; the old privileges were maintained and even augmented by grants of the power of life and death (_hautejustice, blut-bann_). Thus came into existence the class of ecclesiastical princes, who throughout the Middle Ages maintained a state, and wielded a power, comparable with that of any lay feudatory. The ecclesiastical _immunitas_, as early as the ninth century, was in the eyes of all ambitious landowners the model of a privileged estate. But it was by another road that the layman arrived at the position of a petty sovereign. Speaking broadly, there are two stages in his progress. First, he comes into the position of a royal tenant, holding his lands in exchange for services and fealty. Secondly, he acquires, by delegation or usurpation, a greater or smaller part of the royal authority over his own dependents.