John of Gaunt, Edward III's third son, married his mistress Katherine Swynford in January 1396, and Pope Boniface IX issued a Bull in September 1396 acknowledging the marriage and legitimising their four children. Richard II further issued letters patent which was confirmed in parliament in February 1397.

“Their legitimation in 1397 had been unconditional. Yet the enabling act referred to property, office and noble rank. Although it accorded the right to dignities it conferred no royal interest or right and title to the throne. Here the language of the act was significant. It used no words of empire, majesty or sovereignty, such as 'regnum', 'summa potestas', 'corona' or 'maiestas' that would enable a man to be an heir of a kingdom.”

From: The King's Mother: Lady Margaret Beaufort, Countess of Richmond and Derby, p.24. Michael K.Jones, Malcolm G.Underwood. Available from Amazon.co.uk Amazon.com

John Beaufort requested an exemplification in 1407 and, to the original wording, Henry IV added excepta dignitate regali (beside regal), therefore barring any issue of the Beaufort line from the throne.