Duke
2 of the 7 encyclopedias on this shelf carry an entry for Duke. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
in Great Britain, the highest rank in the peerage. The first hereditary duke in England was the Black Prince, created by his father, Edward III., in 1336. The Duchy of Cornwall was bestowed upon him, and was thenceforward attached to the eldest son of the king, who is considered a duke by birth. The Duchy of Lancaster was soon after conferred on Edward's third son, John of Gaunt, and hence arose the special privileges which these two duchies still in part retain. A duke in the British peerage, not of royal rank, is styled "your grace," and is "most noble"; his wife is a duchess. The coronet consists of a richly chased gold circle, having on its upper edge eight golden leaves of a conventional type called strawberry leaves; the cap of crimson velvet is closed at the top with a gold tassel, lined with sarsnet, and turned up with ermine. At various periods and in different continental countries the title duke (Herzog in Germany until the close of the World War) has been given to the actual sovereigns of small states.
Aiton's Encyclopedia (1910)
a title of nobility ranking below that of Prince. The term is akin to the Venetian doge and the French duc. All three are derived from the Latin dux, a military leader. The younger sons of the sovereign of the United Kingdom are dukes. About thirty English dukes sit in the House of Lords by right of birth. The Irish and Scotch dukes select a part of their number. The first English duke was Edward, the Black Prince, who was made Duke of Cornwall in 1337. Dukes are created by the sovereign. On state occasions a duke is permitted to wear a coronet, consisting of a band of fine gold surmounted by eight strawberry leaves of the same metal. The top may or may not be closed by a velvet cap. In Austria the dukes of the royal family are called archdukes. In Russia, they are called grand-dukes. In Germany the title is held only by five persons. Each is the ruler of an independent principality. See Precedence