The bikini was born at a Paris poolside photo shoot on July 5, 1946, a week before Bastille Day and in the midst a global textile shortage. The designer, former engineer Louis Réard, hired the only model willing to expose so much model, a 19-year-old nude dancer from the Casino de Paris named Micheline Bernardini. She put on the four small patches he had strung together and showed the fashion world the female belly button.

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Réard’s innovation wasn’t the first to split women’s traditional swimwear in two. Hollywood icons and pinup models had long worn two-piece suits, as was evident under the lids of thousands of GI footlockers still being shipped home from Europe. But that navel was novel.

Kelly Killoren Bensimon, who recorded a history of the garment in “The Bikini Book,” said that last inch of midriff was fashion’s final “zone of contention.”

“We had seen Jayne Mansfield and a lot of other actresses wearing two-piece bathing suits,” Bensimon said in an interview. “But never with the navel showing. That was the scandal.”

Aside from some artefactual evidence that female athletes competed in two-piece garb in ancient Greece and Rome, the history of women’s bathing costumes is one long coverup. Victorian women who dared to bathe in public did so in long smocks that were only distinguishable from their daily wear by the fact that they were dripping wet.

But hemlines slowly crept up, necklines slipped down and sleeves retracted over the shoulder. Women’s swimming was introduced in the 1912 Olympic Games in Stockholm, and a Portland, Ore., woolen-maker named Carl Jantzen began marketing a sleeveless one-piece that left the legs bare, although his knitted wool garment came with matching cap and stockings.

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Rayon and other new synthetic fabrics made for more comfortable and form-fitting suits, which was fine with Busby Berkeley and other choreographers of the synchronized pool dance numbers that became a movie staple. By the 1940s, swim star Esther Williams was on screens and posters in two-piece suits that showed a swath of above-the-button belly and the abs that had made her a national champion in the 100-meter freestyle.

Shocking stuff, but once the war was finished, the tan lines were destined to become more tangled yet.

By 1946, Réard had left automotive engineering to work in his mother’s lingerie business. In the heady months after the armistice, he was in an arms race with another designer to create the world’s smallest swimsuit.

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The rival, Jacques Heim, claimed success with a design he called “the Atom.” But Reard, stitching together a napkin’s worth of newsprint-patterned fabric, achieved something smaller than Heim’s Atom, which he named after the Bikini Atoll, the remote island where atoms were being split in atomic bomb tests that very week.

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“We’ve seen it after many wars,” Bensimon said. “In the safer time to follow, we get these celebrations of freedom and the human body.”

Part Two (the brief part)

At first, the bikini was more of a sensation than a success. Some photographers and models did dare to shoot the suits, and Réard built his own business around the design.

But it was slow to break through the modesty barriers on European beaches, much less in the postwar United States. Many commentators condemned the look, and plenty of communities banned it. Even today, the swimsuit is the center of debate in some Western locales; Barcelona banned wearing a bikini on the streets in 2011.

But celebrities began their own navel maneuvers. Six years after Bernardini was the first girl in a bikini, Bridgette Bardot made “The Girl in the Bikini.” In 1962, Ursula Andress strode from the surf in “Dr. No” in nothing but a knife and bikini as the original Bond girl.

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It was the Jet Age that really boosted the fortunes of the skimpy garment, according to Bensimon. The wealthy and glamorous began to shuttle in and out of the Riviera, bringing new standards of beachwear to shores — and soon pools — around the world.

In the United States, it took a former Mousketeer to make the big reveal okay for family viewing, Bensimon said. When Annette Funicello, a child star from the Mickey Mouse Club, got permission to wear a bikini in most of her madcap beach movies, it wasn’t just Frankie Avalon who did a double take.

“Everybody noticed,” Bensimon said. “She just wanted to be one of the cool kids.”

So did many of the kids who saw her. By then, America’s top-to-bottom adoption of the bikini was irreversible.