In every social structure that existed since the beginning of human history there is always an small “elite” of leaders which their role is to organize and to rule/command the rest of the people (anarchy excluded). In England of the middle ages (and later on the British Isles) these ranks are populated by the Royal family followed by five ranks of nobility.
It’s all about land
The simplest way to understand the organization of a Feudal kingdom is always to think about the land. In the case of a serf, they belonged to the land, as a Freeman you had the rights to your own piece of land, as a noble you manage a piece of land and finally, as a royal you own the land. The King or Queen has a right to hand over titles, essentially in most cases, redistributing the management of their lands. The nobles managing the land were responsible for the protection of the land but also owned all it’s generated wealth (in form of taxation). In return, they offered part of their wealth to the King and in times of war, military assistance.
A duke, being the highest rank of a noble was granted a duchy from the king to reward him for his fealty and to keep them happy.A duchy was separated into administrative sectors called counties or, in the case of a county at the borders of the kingdom, marches. A Duke, as a Lord of Manor had the right to request the serfs of the land to work without pay in order to complete public works within the duchy.
A Marquess was the lord of a March, a territory at the borders of the kingdom, and as such he would almost certainly be the one responsible to fend off any external threats to the kingdom. Because of that, although the size of the land he owned was comparable to an Earl’s, (lord of a county) he enjoyed a higher title.
Earls, like Marquess, were responsible for the management of a county and as such, for the taxation and protection of their territory.
Viscounts are the only non-landed non-hereditary, title of the kingdom. A Viscount is, in most cases, the first born son of an Earl or a Marquess and it’s honorific title. In a way the title of a Viscount was there to acknowledge someone as noble. Since the titles of Earl and Marquess were hereditary it meant that a Viscount was to become an Earl or Marquess upon the death of their father (or, occasionally their mother)
Barons are essentially Lords of a Manor, which could have been a village, a market town or in very rare occasions, a city.
Nobles in the service of the King
As with a modern corporation the lower ranks did not owe fealty to their direct managers, (A Baron to the Count of it’s territory, a Count to the Duke of his duchy etc) being a noble meant that you swear fealty to the King and that implied yearly military service. Military service meant that themselves and their retinue (private armies) were expected when called by the King to come to their assistance.
The rich and the famous
Each of the nobles owned taxation directly to the King, the taxation came from the Manors and Infrastructure they held directly and was managed either by them or by their Bailiffs. In addition to their direct holdings a noble also received a small portion of the tax generated from the holdings of other Lords within their administrative section. The higher the rank of a noble it was the larger and more numerous the holdings he would manage within his territory. For example the Duke of a duchy would most probably manage the largest settlements (cities) within the region as well as several Manors within it. It was fairly common that a Duke would hold one or more counties as his own as well as Baronies (Manors) as an Earl would hold several baronies within his County.
For example the Duke of X would also be the Earl of XY and the Earl of XZ as well as the Baron of XYa XYb and XZc and he would take money geld (money in the form of tax) from these holdings as well as a small portion of the tax generated by XYc and XZa, XZb. Each year the Baron will have to give to his King tax for all his direct holdings and the Counts of his duchy will also pay the King’s tax directly to the King (minus the portion that was given to their Duke)
In a Feudal kingdom the nobles of the land were men who declared their fealty to their King and received part of the King’s kingdom in return for a portion of their income and military assistance in times of war. Each of these lords (Viscounts excluded) held their title in their family, which meant that one of their sons (firstborn in most cases) would inherit all the land they were given and hold the same obligations to their King.
References and Further reading
Larence, Sir James Henry (1827) [first published 1824]. The nobility of the British Gentry or the political ranks and dignities of the British Empire compared with those on the continent (2nd ed.). London: T.Hookham — Simpkin and Marshall. Retrieved 2013-01-06.