In England's ancient past large parts of the realm were unoccupied[1] and owned as allodial titles: the landowners simply cooperated with the king out of a mutual interest instead of legal obligation.
It was not until the Norman conquest, when William the Conqueror declared himself to be the sole allodial owner of the entire realm, that land tenures changed drastically.
He introduced a new type of feudalism, in which obligation extended right down through the hierarchy, a model informed by the military system.
[4] The tenants-in-chief held their land by the tenure of barony, which required the tenant to provide a number of knights for their liege for 40 days per annum.
A few estates retained the same wealth and population as when first enfeoffed, with the result that the lord provided only a small number of the knights whom he was actually able to muster.