Before 01 Oct 1369 [his father] Robert Willoughby 4th Baron Willoughby (age 20) and [his mother] Margery Zouche Baroness Willoughby of Eresby (age 30) were married.

After 03 Jan 1383 William Willoughby 5th Baron Willoughby (age 13) and Lucy Strange Baroness Willoughby Eresby were married. They were third cousins.

Before 1387 [his son-in-law] Henry Beaumont 5th Baron Beaumont (age 7) and [his daughter] Elizabeth Willoughby Baroness Beaumont (age 2) were married. She by marriage Baroness Beaumont. They were third cousin once removed. He a great x 4 grandson of King Henry III of England.

After 17 Oct 1388 [his father] Robert Willoughby 4th Baron Willoughby (age 39) and [his step-mother] Elizabeth Latimer 5th Baroness Latimer, Baroness Neville and Willoughby (age 31) were married. She by marriage Baroness Willoughby de Eresby.

On 04 Nov 1393 Edmund of Langley 1st Duke of York (age 52) and [his future wife] Joan Holland Duchess York (age 13) were married. She by marriage Duchess York. The difference in their ages was 38 years. She the daughter of Thomas Holland 2nd Earl Kent (age 43) and [his future mother-in-law] Alice Fitzalan Countess Kent (age 43). He the son of King Edward III of England and Philippa of Hainault Queen Consort England. They were half second cousin once removed. She a great x 2 granddaughter of King Edward I of England.

Before Jul 1396 [his brother] Thomas Willoughby (age 16) and Elizabeth Neville (age 12) were married. They were step-brother and sister since his father [his father] Robert Willoughby 4th Baron Willoughby (age 47) married her mother [his step-mother] Elizabeth Latimer 5th Baroness Latimer, Baroness Neville and Willoughby in 1388. She a great x 5 granddaughter of King Henry "Curtmantle" II of England.

Before 09 Aug 1404 William Willoughby 5th Baron Willoughby (age 34) and Joan Holland Duchess York (age 24) were married. She by marriage Baroness Willoughby de Eresby. She the daughter of Thomas Holland 2nd Earl Kent and Alice Fitzalan Countess Kent (age 54). She a great x 2 granddaughter of King Edward I of England.

Before 18 Nov 1406 [his son-in-law] William Fitzhugh 4th Baron Fitzhugh (age 7) and [his daughter] Margery Willoughby Baroness Fitzhugh (age 9) were married.

After 06 Sep 1410 Henry Scrope 3rd Baron Scrope of Masham (age 37) and [his former wife] Joan Holland Duchess York (age 30) were married. She by marriage Baroness Scrope of Masham. She the daughter of Thomas Holland 2nd Earl Kent and [his former mother-in-law] Alice Fitzalan Countess Kent (age 60). She a great x 2 granddaughter of King Edward I of England.

Before 27 Apr 1416 Henry Bromflete 1st Baron Vesci (age 9) and [his former wife] Joan Holland Duchess York (age 36) were married. There appears to be a twenty-seven year age gap here which may suggest the marriage ewas a reward to him since she, having being married three times before, including to Edmund of Langley 1st Duke of York, would be a wealthy widow. The difference in their ages was 26 years; she, unusually, being older than him. She the daughter of Thomas Holland 2nd Earl Kent and [his former mother-in-law] Alice Fitzalan Countess Kent (deceased). She a great x 2 granddaughter of King Edward I of England.