* No matter what drama might be going on between various members of the royal family, the Queen will never say a bad word about any of her relatives (or in-laws) to anyone outside the family itself.

* In his book Long Live the Queen! 13 Rules for Living from Britain's Longest Reigning Monarch, royal author Bryan Kozlowski explains why the Queen prioritizes keeping her family strong over everything else.

* Louis Mountbatten once said that keeping the royal family together in spite of all their drama was actually the Queen's greatest achievement, since their difficulties are "always the focus of public attention."

The Queen has a strict rule that she always follows: Never talk badly about any member of the royal family to outsiders—no matter what.

It's not an official edict, but this unspoken rule has been important to the monarch, particularly as she works to keep her family strong even as the world watches their every move (and gossips endlessly about every feud or bit of drama they go through).

The rule is one that royal author Bryan Kozlowski outlines in his book Long Live the Queen! 13 Rules for Living from Britain's Longest Reigning Monarch, which guides readers through how to live a happy and successful life by taking tips directly from the Queen herself.

"One of the Queen's unspoken rules is to never talk badly about any member of her family to outsiders, rather only support them, when she can," Kozlowski writes, citing Princess Diana's ongoing drama in the 1990s as example of a time when the Queen refused to deviate from her supportive stance, even when it wasn't the easiest thing to do.

"Diana might have made her life unimaginably difficult in the 1990s, but the Queen quickly jumped to her defense when, during a press conference at the Palace, an editor of the News of the World, inquired that if Diana didn't want the paparazzi following her, couldn't she send a servant to run her errands for her?" he writes. "'That's the most pompous thing I have ever heard,' responded the Queen, famously earning a round of applause from the other editors in the room."

Over the years, some people, including Prince Philip's uncle, Louis Mountbatten, have taken note of the Queen's stalwart support for her family members and praised her ability to handle all of their drama and difficulties with grace.

"The more he witnessed the royal drama unfold, the more Louis Mountbatten believed that keeping her family together and coping with their self-inflicted messes was the Queen's greatest personal achievement," Kozlowski writes. "More so when you consider, as Mountbatten told his confidant John Barratt, that while 'most people can hide their family difficulties...hers are always the focus of public attention.' Yet, for the Queen, it's always been the bigger picture, not the isolated tiffs with her children, grandchildren, husband or sister, that have most concerned her. She still believes, as her mother did before her, that 'a united family is the strongest thing in the world.'"