Prince Andrew and Sarah Ferguson have been divorced for over two decades. But until today, they have stayed friends.

While speaking with Express, relationship expert Martina Mercer said that even though it’s quite uncommon for former couples to be friends after a divorce it is not necessarily unheard of. In fact, some former couples also end up as best friends.

The Duke and Duchess of York are no exception. And according to Mercer, ex-couples who are able to stay friends have a certain level of maturity. And it is likely that their main priority is their children.

“This type of relationship is usually born out of a desire to make the children’s lives as stress-free as possible, while it can also signify Sarah’s and Prince Andrew’s reluctance to let a good relationship go,” she said.

However, Mercer also said that there may be some downsides to former couples being friends after their divorce. As such, there is a need to put some boundaries between the parties involved.

“For it [their relationship] to work, there generally needs to be no physical attraction left, as this causes resentment in the long run as one feels their lust is unreciprocated. Although it’s ideal for the children it can make things difficult when either party wants to move on with a new partner. They may have an agreement that they don’t bring anyone else into the children’s lives for a set number of years or they may agree not to replace the other’s parenting role with a new partner,” she said.

Mercer added that Prince Andrew and Ferguson’s future partners should be open to the idea of having the Duke and Duchess of York around. The former couple’s future partners should also have a good relationship with Prince Andrew and Ferguson.

The royal couple’s future partners should also have a certain level of maturity because they may feel like a third wheel in Princess Beatrice and Princess Eugenie’s parents’ unique relationship.

Prince Andrew, Sarah Ferguson
Pictured: Prince Andrew, Ferguson attend the Virgin London Marathon on April 25, 2010 in London, England. on April 25, 2010 in London, England. Getty Images/Gareth Cattermole