It was just after 9 a.m. on September 8, and the weather in New York City was perfect.

The sun was shining.

The temperature hovered at around 70 degrees.

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Photos: Anwar Hussein / Times Newspapers / Shutterstock

The dew points were low.

But this particular outfit?

I can tell youexactlywhere it came from:Princess Diana.

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Princess Diana after leaving a gym in Earl’s Court.

How did it catch on?

Yet, her athleisure looks also have a fascinating history.

Princess Diana never liked being photographed in her exercise clothesand even took great lengths to avoid doing so.

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Princess Diana leaving Chelsea Harbour Club in 1995.

(The Princess later sought legal action.)

But her greatest hack?

Shed wear thesame outfitevery time: spandex bike shorts with a sweatshirt, again and again.

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Princess Diana leaving Chelsea Harbour Club in 1994.

It wasnt her that asked for all this.

A side effect of her dedication to sartorial repetition is that the style became synonymous with Diana herself.

During the 2010s, internet and social media culture reached its full power.

Suddenly, Dianas 90s gym clothes felt downrightfashionable.

The images went viral.

So go ahead, throw on your favorite sweatshirt, and head out the door.

It would be rather regal of you.