Five degrees of hereditary nobility arranged in precise order of precedence — from the Duke who rules a territory as vast as a small country to the Baron who holds a single manor. The spine of the British aristocratic order since the Norman Conquest, and the model that every subsequent hierarchy — including Goetia's spirit ranks — has followed.
The British peerage developed in parallel with — and in constant dialogue with — the noble hierarchies of continental Europe. The titles are often cognate (derived from the same Latin or Germanic roots) but the specific powers, duties and precedence varied significantly between countries. What was a Duke in France was not the same political reality as a Duke in England.
The House of Lords was, for centuries, the assembly of the peerage — the chamber where the great hereditary nobles of England sat as a matter of right, debating and passing legislation alongside (and often against) the elected House of Commons. The Lords' power rested on their land, their wealth and the principle that those with most to lose in a well-ordered society had the greatest stake in its governance.
The 72 spirits of the Ars Goetia — the great medieval grimoire of demonic hierarchy — are ranked using exactly the titles of the noble peerage: Kings, Dukes, Marquises (Marquesses), Earls, Presidents, Knights and Barons. This is not coincidence. The grimoires were composed in the same medieval world that created and used the peerage system; their authors naturally mapped supernatural hierarchy onto the most elaborate earthly hierarchy they knew.
Of the 72 spirits: 4 are Kings (ruling 36 legions each), 30 are Dukes (ruling between 3 and 40 legions), 8 are Marquises, 3 are Earls, 11 are Presidents, 3 are Knights and 11 are Counts/Earls. The distribution is interesting — Dukes are the most numerous class, suggesting that the grimoire authors associated the management of supernatural forces with the middling aristocracy rather than the highest ranks. The King spirits command the most legions but the Dukes do most of the actual work — which, perhaps, is a remarkably accurate description of how medieval feudalism actually functioned.
"The first principal spirit is a King ruling in the East, called Bael. He maketh thee to go Invisible. He ruleth over 66 Legions of Infernal Spirits."
— Ars Goetia, Lesser Key of Solomon, 17th century