As far back as 2004, when male celebrities and fashion editors were spotted letting their shirttails flap proudly in the breeze, the untucked, button-down shirt seemed like a curious trend, born of Casual Fridays and Silicon Valley tech startups. But trends — like mullets, velour tracksuits, MC Hammer pants — have short life spans.

Yet, here we are, 13 years later, and the untucked shirt phenomenon is not only still with us but growing.

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“I always wore my shirts untucked, even when I was 14, 15 years old,” says Chris Riccobono, who in 2011 co-founded UntuckIt, a leading brand of shirts designed to be worn . . . well, you get the idea.

The problem with untucked shirts — and the reason many guys look at their untucked selves in the mirror and wonder, “Why doesn’t this look right?” — is that most dress shirts aren’t designed to be worn untucked.

“They’re too long, too baggy,” says Riccobono, who — after 31 prototypes — came up with a modified shirt design, where the bottom hem hits in the middle of a guy’s fly, with pants pockets just partially exposed.

Made-to-measure companies are starting to take this into account, too. J. Hilburn’s personal stylists (who help you custom-design shirts, plus jackets, pants and a whole wardrobe) will ask how you like to wear your shirts, and Proper Cloth (which customizes shirt designs based on how clients answer 11 questions online) will adjust the size of your shirt just a tad if you plan to wear it untucked.

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“Walk into the highest-priced restaurants in New York on a Friday night, and you’ll see guys in untucked shirts,” says Riccobono. “The world’s become extremely casual — which is great for us.”