As I uncover the extended familial relationships of the Barons of 1215, I am frequently surprised by finding cross-generational support for Magna Carta. A son or son-in-law might be joined on the fateful field by the Thames River by his wife's father or perhaps even her grand-father.
The most stressful relationship of all must have fallen to Sir William Marshall, Fourth Earl of Pembroke and King John's "Marshall of England", standing loyally by King John, and William Marshall Jr., Fifth Earl of Pembroke, who was one of the twenty-five Barons of the Sureties of Magna Carta. King John must not have held a grudge, because he later gave William, Jr. the office his father had held, Marshall of England, and twelve years after Magna Carta, King John gave William Jr his daughter, Eleanor of England, in marriage.
So far I have identified four of my ancestors among the gathering at Runnymede.