Prince Philip and Queen Elizabeth (opens in new tab) were married for 73 years until Queen Elizabeth's death earlier this month, and Philip knew exactly why their marriage worked.

In his book, "Long Live the Queen: 13 Rules for Living from Britain's Longest Reigning Monarch" (opens in new tab), royal author Brian Kozlowski (opens in new tab), Elizabeth and Philip s long and happy marriage.

According to Kozlowski, Philip readily admitted that "generosity is the essential ingredient" for maintaining a healthy and happy marriage with his wife.

In Philip and Elizabeth's marriage, the Duke of Edinburgh, famed for his temper, showed no signs of tolerance. As he explained, "Queen Elizabeth has the qualities of tolerance in spades."

Kozlowski described this as the Queen's ability to "strategically distance herself from her family" (open in new tab).

"Philip may have been the queen's "power and place" for over 70 years, but he was also irritating, irascible, critical, moody, cruel, and did not hesitate to call the queen a "bloody fool" when she got under his skin," Kozlovsky wrote in Philip explained in a book published before his death. Says biographer Carolee Erickson. 'I will not appear until Philip is in a better mood,' the queen would say"

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The Queen's ability to recognize when Philip (opens in new tab) needed that space, and to tolerate his moodiness in the first place, was clearly an important element in their 73-year marriage. Our thoughts are with the Queen and the rest of the Royal Family as they mourn Philip's passing.