Bizarre detail about Bill and Melinda Gates' marriage revealed days after announcing divorce

A bizarre detail about the marriage of Bill and Melinda Gates has emerged just days after the couple announced their divorce.

Each year, Gates would spend a long weekend away with his ex-girlfriend Ann Winblad who went on to marry the brother of actor Kevin Kline, Alex Kline.

Speaking to the New York Post, a source said Gates and Winblad remained close friends, despite ending their relationship and Winblad gave her seal of approval before Gates proposed to Melinda.

At the core of Gates' friendship with Winblad was biotechnology, Gates told Time magazine in 1997.

"We can play putt-putt while discussing biotechnology," Gates said.

"When I was off on my own thinking about marrying Melinda, I called Ann and asked for her approval."

Gates and Winblad dated from 1984 and after meeting at a computer conference and continued taking their annual trips during which they would hand glide, ride dune buggies and walk along the beach. They also travelled overseas including to Indonesia and Germany.

"We share our thoughts about the world and ourselves," Winblad told Time.

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"And we marvel about how, as two young overachievers, we began a great adventure on the fringes of a little-known industry and it landed us at the center of an amazing universe."

Bill and Melinda Gates were married in 1994 and remained together for 27 years. Their divorce will be one of the most expensive in history, worth an estimated $130 billion ($AUD 168 billion) according to Forbes.

The couple didn't have a prenuptial agreement.

They have three children - Jennifer, 25, Rory, 21, and Phoebe, 18.