General Notes

Sir John Beaufort (Beaufort being the name of a castle in Champagne belonging to his father), KG (c1397); created 10 Feb 1396/7 Earl of Somerset and 29 Sep 1397 Marquess of Dorset and Marquess of Somerset (he was degraded from both marquessates 1399 following accession of Henry IV), Constable of England 1404.  [Burke's Peerage]

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John Beaufort, Earl of Somerset, Marquis of Dorset, KG, b. c 1372, d. 16 Mar 1410.  He was legitimated with the 3 other children of Katharine by Act of Parliament 20 Richard II (1397) for all purposes except the royal succession.  He married as her 1st husband, c 1399, Margaret, d. c 30 Dec 1439, 3rd daughter and eventual coheir of Thomas Holand, 2nd Earl of Kent, KG.  She m. (2) 1411 Thomas, Duke of Clarence, KG, 2nd son of Henry IV.  [Magna Charta Sureties]

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On the Dukedom of Somerset [Burke's Peerage, p. 1661]

Two and a half ceturies later Richard II created the eldest of John of Gaunt's bastards by Catherine Swynford, John Beaufort, Earl of Somerset.  He was later promoted Marquess of Somerset, though degraded from the rank in the early part of Henry IV's reign.  Both before and after the deposition of Richard he was a prominent military figure, chiefly in France during the Hundred Years War.  The latter conflict continured to provide the Beafort Earls of Somerset with opportunities; the 3rd Earl [John Beaufort], one of John's younger sons, was promoted Duke of Somerset in 1443 following various military operations in Normandy, Anjou, and Maine, though these were competently executed rather than conspicuously triumphant.  Probably his close kinship with Henry VI was the chief cause of his leap in the peerage, for his mother Margaret was daughter of Thomas Earl of Kent, a descendant of Edward I.  It is this Duke of Somerset's daughter who was mother of Henry VII.

The Dukedom of Somerset created in 1443 expired with the death of the grantee [John Beaufort] in 1444.