"Joan Beaufort, Countess of Westmorland (c. 1379 – 13 November 1440) was the third or fourth child (and only daughter) of John of Gaunt, 1st Duke of Lancaster and his mistress, later wife, Katherine Swynford; and, in her widowhood, a powerful landowner in the North of England."

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Links:

http://en.wikipedia.org/wiki/Joan_Beaufort,_Countess_of_Westmorland

http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=16146333

http://www.geneall.net/U/per_page.php?id=1302

http://histfam.familysearch.org/getperson.php?personID=I467&tree=EuropeRoyalNobleHous

http://www.mathematical.com/beaufortjoan1375.htm

http://thepeerage.com/p10198.htm#i101973 

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Citations:

1.[S11] Alison Weir, Britain's Royal Family: A Complete Genealogy (London, U.K.: The Bodley Head, 1999), page 108. Hereinafter cited as Britain's Royal Family.

2. [S6] G.E. Cokayne; with Vicary Gibbs, H.A. Doubleday, Geoffrey H. White, Duncan Warrand and Lord Howard de Walden, editors, The Complete Peerage of England, Scotland, Ireland, Great Britain and the United Kingdom, Extant, Extinct or Dormant, new ed., 13 volumes in 14 (1910-1959; reprint in 6 volumes, Gloucester, U.K.: Alan Sutton Publishing, 2000), volume I, page 27. Hereinafter cited as The Complete Peerage.

3. [S125] Richard Glanville-Brown, online <e-mail address>, Richard Glanville-Brown (RR 2, Milton, Ontario, Canada), downloaded 17 August 2005.

4. [S11] Alison Weir, Britain's Royal Family, page 109.

5. [S6] Cokayne, and others, The Complete Peerage, volume XII/2, page 547.

6. [S8] Charles Mosley, editor, Burke's Peerage and Baronetage, 106th edition, 2 volumes (Crans, Switzerland: Burke's Peerage (Genealogical Books) Ltd, 1999), volume 1, page 17. Hereinafter cited as Burke's Peerage and Baronetage, 106th edition.

7. [S8] Charles Mosley, Burke's Peerage and Baronetage, 106th edition, volume 1, page 15.