Inside Prince Phillip And Queen Elizabeth's Marriage 

longest royal marriage in history – and that’s because Queen Elizabeth, 91, and Prince Philip, who turns 96 in June and announced his retirement Thursday, have an ideal dynamic, says their biographer Gyles Brandreth.

, offers insight into the royal couple’s 68-year union.

Here’s what makes their remarkable marriage work:

With his dry wit and famously uncensored humor, Philip has long been the person who can most bring a smile to the Queen’s face. During a visit to Wales on Tuesday, he showed his humor once again in an exchange with 10-year-old Katrin Powell.

“He asked us if we could speak Welsh, and we said yes,” she told reporters. “He said, ‘You must have really good brains to speak Welsh.’ ”

Want to keep up with the latest royals coverage? Click here to subscribe to the Royals Newsletter.

“The Duke of Edinburgh is a funny man,” Brandreth wrote in

. “He is the fellow who said, ‘If you ever see a man opening the car door for his wife, it’s either a new car or a new wife.’ He has been making the Queen laugh for more than 70 years.”

Since vowing at her 1953 coronation to be the Queen’s “liege man of life and limb,” Philip has dutifully fulfilled her role as the Queen’s consort – but that doesn’t mean he takes a back seat in their relationship.

“The Queen wears the crown, but her husband wears the trousers,” writes Brandreth. “He is the power behind the throne: steadfast, never-failingly supportive.”

RELATED VIDEO: Here are Some of Prince Philip’s Most Memorable Lines Over the Years

What’s more, “because she is the Queen, no one in the world treats Elizabeth II entirely normally – except for Prince Philip,” says the biographer. “Conversely, the Queen is the only person in the world who can say to the Duke of Edinburgh, ‘Oh Philip, do shut up.’ And she does.”

Philip, who “wants to do what he did when he was 55”, his grandson Peter Phillips recently said, “is dynamic, outgoing, adventurous, challenging,” observes Brandreth. The Queen, on the other hand, “is conservative, placid, content to do things as they have been done before. He reads a good deal, where she doesn’t. He has infinite patience. He has rather less . . . As a couple they are allies, but that doesn’t mean they don’t have their differences.”

1 Prince Philip, 95, Retires from Royal Duties After 70 Years by Queen Elizabeth’s Side

2 Why Prince Charles Is ‘Pragmatic’ About Not Seeing George and Charlotte as Much as They See the Middletons

3 Queen Elizabeth and Prince Philip Are Just Fine, Thanks for Asking

4 How the Queen Fought to Marry the Love of Her Life, Prince Philip

5 WATCH: Prince Philip Has the Perfect Comeback to Rumors of His Decline