Montesquieu

When in a monar­chy the nobi­lity has the land tilled for its bene­fit by the conque­red peo­ple, again the debt must not be sub­ject to increase.1 Moreover, it is good for the prince content him­self with his domain and mili­tary ser­vice. But if he wishes to levy tri­bu­tes in spe­cie on the sla­ves of his nobi­lity, the lord must be the gua­ran­tor of the tri­bute2 : he must pay it for the sla­ves and col­lect it back from them ; and if this rule is not fol­lo­wed, the lords and those who levy the prince’s tri­bute will extort the slave by turns, and harass him one after the other until he dies of misery or flees into the woods.

That is what led Charlemagne to make his fine Institutions on that basis. See book V of the Capitulaires, art. 303.

It is so practiced in Germany.