And now for my theory. No one can have even glanced at the lists I have compiled without being instantly struck by the fact that the 'service' is reckoned in round numbers, and is almost invariably _a multiple of 5, if not of 10_.[96] This discovery, of course, is absolutely destructive of the view that it always represented the number of five-hide (or £20) units contained in the fief. Further, the number of differing fiefs assessed at precisely the same figure proves that the assessment was wholly arbitrary and cannot have been even the round sum which approximated most nearly the number of such units.[97] What then was the true determinant in the light of these conclusions? I reply--_the unit of the feudal host_. 'On the continent,' writes Gneist, 'fifty _milites_, or at least twenty-five, were reckoned to one banneret; in England, in proportion to the smaller scale of enfeoffments, a smaller number appears to have formed the unit of the _constabularia_.'[98] He is right: the English _constabularia_, where I find it referred to, consists of _ten_ knights.[99] It is interesting to trace this unit and its multiples recurring in the narratives of Irish warfare, under Henry II, and in other struggles.[100] We meet with it also in the grant by the Empress to Geoffrey de Mandeville, in 1141, of 'feodum et servicium xx. militum' and in Stephen's grant to him of 'lx milites feudatos'.[101] The next step is to show that the Normans were familiar with _servitium debitum_ in terms of the ten-knight unit when they landed in England. For this we have only to refer to Wace. For in the 'Roman de Rou', as quoted by Mr Freeman himself, we find William fitz Osbern assuring the duke as to his barons: Vostre servise dobleront: Ki solt mener vint chevaliers Quarante en merra volontiers, E ki de trente servir deit De sesante servir vos velt, E cil ki solt servir de cent Dous cent en merra bonement.[102] The _servitium debitum_, therefore, was a standing institution in Normandy, and 'to the mass of his (William's) followers', as Mr Freeman frankly admits,[103] a 'feudal tenure, a military tenure, must have seemed the natural and universal way of holding land'. When we find them and their descendants holding their fiefs in England, as they had been held in Normandy, by the service of a round number of knights, what is the simple and obvious inference but that, just as Henry II granted out the provinces of Ireland to be held as fiefs by the familiar service of a round number of knights,[104] so Duke William granted out the fiefs he formed in England?