A whole French dynasty stepped inside the majestic military office, soon to be the Saint Laurent headquarters, where Anthony Vaccarello staged his debut as the new creative director for YSL. Their castle: a one-time abbey on Paris's Left Bank, its façade crumbling, its interior gutted. Their throne: front-row seats to Vaccarello's much-anticipated first show in the post-Hedi Slimane era. There's the queen mother Jane Birkin, once muse to Serge Gainsbourg and all-around French It girl, now face of Saint Laurent's Le Smoking campaign. Her daughters, Lou Doillon and Charlotte Gainsbourg, contemporary It girls to rival their mother, flanked her. Doillon, noted fan of Saint Laurent boots and a good tuxedo, wore a Saint Laurent Le Smoking. Meanwhile, Gainsbourg, in Saint Laurent boots, leather skirt, and white tee, perfected that nonchalant ease of a cool French girl.

Charlotte Gainsbourg front-row at the Saint Laurent Spring 2017 show in Paris, France, September 2016.

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Who: Charlotte Gainsbourg.

When: Tuesday, September 27.

Where: Front row at Anthony Vaccarello's debut show for Saint Laurent Spring 2017 in Paris, France.

What: A leather miniskirt with a zipper up the front, white tee-shirt with a black bra exposed underneath, and black patent boots, by Saint Laurent.

Why: There's a commonly known truth about French women, that they can wear looks that should not be attempted by mere mortals. Polka-dotted scarf tie? No problem. Overwhelming ruffles, clogs, and sneakers? Sure thing. So that's why Charlotte Gainsbourg's relatively pedestrian Saint Laurent look — simple, black-and-white, with messy hair and minimal makeup — has caught our eye this morning. It works because she's wearing it. Gainsbourg has a Balenciaga scent crafted in her honor and a clothing line with Current/Elliott, and it was only a matter of time before the quintessential Left Bank brand Saint Laurent pulled her into its fold. (Disregarding for a moment that Gainsbourg has long been the muse of another, much older, quintessential Parisian label, Louis Vuitton, due in part to her longtime friendship with creative director Nicolas Ghesquière.) Her look has always been effortless, and here, she capitalizes on that. When you're the muse daughter of a muse, you really don't even have to try.

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