For most writers of historical romance, the "mother of all dynasties" is the English royal family. Many of us have a vague notion of the medieval segment of the story: the Plantagenets, Lancaster, York, and the War of the Roses, the Tudors, and finally, the Stuarts/Stewarts from Scotland after Queen Elizabeth died childless.
The official family tree is hard to follow, but if you study the genealogy, you will discover that the Lancasters, the Yorks, the Tudors, and the Stewarts all have a direct and clear line of inheritance back to the Beauforts, the children of one of the Middle Ages' most romantic couples: Katherine Swynford and John of Gaunt, Duke of Lancaster.
Many readers are familiar with Katherine Swynford because of Anya Seton's Katherine, published in 1954 and still in print today. It fictionalizes the love story of Katherine and John, who was a younger son of Edward III. (Now Katherine has finally gotten her due as two biographies have recently been published: Mistress of the Monarchy by Alison Wier, and Katherine Swynford: The History of a Medieval Mistress by Jeanne Lucraft.)
Among the facts we know is that John and Katherine were lovers for many years, she bore him four children, and the two finally married very late in life. After their marriage, their children, called the "Beauforts" after a French castle John claimed but did not hold, were legitimized. (It is a myth that the children were born there. Neither John nor Katherine ever set foot in the castle.)
Katherine was governess to Gaunt's children by his first two wives and most evidence suggests the siblings of the blended families (her children by her first husband, his children by his previous wives, and their bastard children) got on well.
John's son with his first wife, Blanche, Duchess of Lancaster, became Henry IV of England and the founder of the Lancaster faction in the later War of the Roses. John and Katherine's second son, Henry Beaufort, held the post of Chancellor of England under Henry IV for a time, and subsequently served as Chancellor for his son and grandson, Henry V and Henry VI.
John, the eldest Beaufort son, held the title of Marquess and Earl of Somerset. Somerset served Henry IV, his half-brother, on several diplomatic and military missions. (His shield is pictured above.) Along with him, he often took Thomas Swynford, Katherine's son with her first husband.
The other two first generation Beaufort children were Thomas, who became Duke of Exeter, and Jean, whose second husband was Ralph Neville, Earl of Westmoreland. The Yorkist kings descended from the Neville family, who have been known by history as "Kingmakers."
So the legitimized children took their rightful, active places in the power structure of England. But in 1407, new words were inserted into the Parliamentary document that had legitimized the them ten years before. The addition read that the Beauforts had all rights excepta dignitate regali, that is, all except the royal rights of succession.
Depending on which biography you read, these words were added by Henry IV or by his Council, but regardless, because the change was never ratified by Parliament, they were conveniently forgotten, or ignored, in the years to come.
The full story of the Beaufort family is too long to recount here. They were in and out of favor over the years, but when you follow Katherine and John's descendants, the path of royal succession is clear and direct.
-- From John's son by his first wife Blanche, Henry IV, comes the line of Lancaster (Red Rose) kings. Through Blanche, John held the title of the Duke of Lancaster.
-- From Katherine and John's daughter, Jean, you have a direct line to two Yorkist Kings: Edward IV and Richard III, both of whom were Katherine and John's great-grandsons. Their great-great-granddaughter, Elizabeth of York, was the first Tudor queen.
-- From Katherine and John's oldest Beaufort son, John, you have a direct line to the first Tudor King, Henry VII. So the founders of the Tudor line, Henry VII and Elizabeth of York, were both great-great-grandchildren of John and Katherine! In fact, this was clearly recognized at the time and because of consanguinity, the two had to get papal dispensation to marry.
-- Joan, who was John and Katherine's granddaughter, married James I of the Scottish Stewart kings. So when James VI of Scotland became James I of England many years later, it was, again, a direct Beaufort descendant who took the throne.
So today's English royal family name might more accurately be not Windsor, but Beaufort!