Margaret Beaufort (May 31 1443 - June 29 1509) married four times and had only one child who became Henry VII of England. She was the daughter of John Beaufort, 1st Duke of Somerset, granddaughter of John Beaufort, 1st Earl of Somerset and great-granddaughter of John of Gaunt and his mistress Katherine Swynford; following Gaunt's marriage to Katherine, their children (the Beauforts) were legitimized, but their descendants were barred from ever inheriting the throne.
Margaret's first marriage, to John de la Pole, took place in 1450, when she was still a child, but was annulled after a short time. Her second cousin Henry VI had as yet no children, and considered naming her his heir. He married her to his half-brother, Edmund Tudor, Earl of Richmond. Edmund was the eldest son of the king's mother, dowager Queen Catherine (the widow of Henry V) by her illegal (and so legally invalid) second marriage to a Welsh squire in her household, Owen Tudor. Thus, in one of the great ironies of history, Margaret's son Henry, the Lancastrian claimant to the throne at the end of the Wars of the Roses -- the one who won it all and united the two houses by marrying the Yorkist princess Elizabeth of York -- had plenty of royal blood but no legal claim to the throne.
- A short profile of Margaret alongside other influential women of her time: http://www.guide2womenleaders.com/womeninpower/Womeninpower1450.htm
- An electronic version of Lady Margaret: A Memoir of Lady Margaret Beaufort, Countess of Richmond & Derby, Mother of Henry VII, by E.M.G. Routh, originally published in 1924: http://tudorhistory.org/secondary/beaufort/contents.html