The fitting room at 5 Avenue Marceau, sometime in 1965

The fitting room at 5 Avenue Marceau, sometime in 1965. Yves Saint Laurent kneels beside a model, pins between his lips, adjusting the hem of a dress that will not photograph well but will move beautifully. He does not design for the camera. He designs for the woman who wears the garment after the show, after the fittings, in the small hours when no one is watching. That principle guided every collection for forty years, and it required women who understood the difference.

Three muses shaped Saint Laurent's vision more than any other: Betty Catroux, Loulou de la Falaise, and Catherine Deneuve. Not models in the professional sense—though Deneuve worked that way early on—but collaborators who brought their own codes to the atelier and left with something neither party could have made alone.

Betty Catroux and the invention of le smoking

Betty Catroux met Saint Laurent in 1967 through a mutual friend. She was Brazilian-American, married to a decorator, and possessed the sort of androgynous frame that looked better in men's tailoring than most men did. Saint Laurent noticed immediately. Within a year she was wearing prototypes of what would become le smoking, the tuxedo jacket and trouser combination that remains the house's most plagiarised silhouette.

The jacket itself—single-button, satin-faced peak lapel, nipped at the waist—was not revolutionary in construction. Tailors had been cutting women's evening jackets since the Edwardians, and Marlene Dietrich wore trousers to premieres in the thirties. What Saint Laurent understood, and what Catroux embodied, was that the smoking worked only if the woman wearing it looked more comfortable than the men around her. Dietrich performed masculinity. Catroux simply inhabited it.

She wore the jacket open over bare skin. She wore it with a silk blouse and a necktie knotted like a schoolboy's. She wore it to dinner at La Coupole and to the collections at the Louvre, and in every photograph from that period she looks as though she dressed in under three minutes. That insouciance was the point. Saint Laurent's tailoring was precise—16oz barathea for the trousers, grosgrain for the stripe, silk satin for the lapel facing—but the effect had to read as effortless. Catroux made it look easy because for her it was.

She never modelled professionally. She never walked a runway for Saint Laurent. But every smoking jacket the house produced between 1966 and 2002 was cut with her proportions in mind: narrow through the shoulder, elongated in the sleeve, skimming rather than hugging the hip. When Hedi Slimane revived the silhouette in 2013 under the Saint Laurent Paris label, he built it on the same block.

Loulou de la Falaise and the atelier

Loulou de la Falaise arrived at Saint Laurent in 1972, initially to design accessories. Within two years she was in the studio during fittings, sketching jewellery on the margins of toiles, suggesting a coral bead where Saint Laurent had planned jet. Her influence was less about silhouette than about ornamentation—the way a belt sat on the hip, the scale of a cuff, the particular shade of fuchsia that worked against aubergine.

She came from Anglo-French aristocracy and dressed like a magpie. Her personal wardrobe combined Savile Row tweeds, Indian saris, costume jewellery from the Paris flea markets, and whatever Saint Laurent had sent over that season. The effect should have been chaotic. Instead it looked inevitable, and Saint Laurent began designing with that layered, magpie sensibility in mind.

The haute couture collections from the mid-seventies onward—particularly the Ballets Russes line in 1976 and the Opéra-Ballets Russes collection in 1980—carry her fingerprints. Those were the seasons when Saint Laurent moved furthest from the spare modernism of the smoking and into something more maximal: embroidered boleros, brocade knickerbockers, tasselled silk blouses that referenced both Diaghilev and the Left Bank in a single gesture. De la Falaise styled many of those pieces herself, and her approach was to pile on references until they stopped reading as costume and started reading as character.

She stayed with the house until Saint Laurent's retirement in 2002, longer than any other collaborator. In interviews—she gave few—she described the work as a conversation, not a commission. Saint Laurent would drape a length of cloth on a dress form, and she would add a belt or a brooch or suggest cutting the sleeve shorter. The next day he would present a revised version that incorporated her note and moved three steps further. That iterative process, conducted almost entirely without words, produced some of the house's most intricate work.

Catherine Deneuve and the customer

Catherine Deneuve met Saint Laurent in 1965, before Belle de Jour, before the smoking, before Saint Laurent had fully separated his vision from the Dior atelier where he trained. She was twenty-one and already working steadily in French cinema. He was twenty-eight and about to stage his first independent collection. They understood each other immediately.

Deneuve became the house's ideal customer: someone who wore the clothes in public, in contexts that had nothing to do with fashion, and made them look like the only reasonable option. She wore Saint Laurent to film premieres, to Cannes, to dinner, to the sort of provincial openings where most actresses would have chosen something safer. She wore the smoking to the Césars in 1977, when no one else in the room was in trousers. She wore his day dresses on talk shows and his evening coats to the opera, and in every instance the garment looked like an extension of her own taste rather than a designer's imposition.

Saint Laurent dressed her for Belle de Jour in 1967, the Buñuel film that remains the clearest distillation of his aesthetic. The costumes—crocodile handbags, patent trench coats, those severe little day dresses in navy and black—were not glamorous in the conventional sense. They were the clothes of a woman who had thought carefully about what she wanted to project and then dressed to match. That quality, the sense of considered restraint, became Deneuve's signature and Saint Laurent's.

She continued to wear the house long after Saint Laurent retired, through the Tom Ford years and into the Slimane era. In photographs from the 2010s she is still in a smoking jacket, still in a silk blouse and high-waisted trousers, still making it look as though she has been dressing this way since birth. Perhaps she has.

What remains

Saint Laurent closed his atelier in 2002. De la Falaise died in 2011. Catroux still lives in Paris and still wears the smoking, now cut by Anthony Vaccarello. Deneuve wears Vaccarello too, though less frequently than she wore the original.

The house continues. The silhouettes continue. But the collaboration—the sense that designer and muse were building something together, in real time, without a marketing brief—belongs to a different era. Vaccarello designs well. He understands the codes. But he designs for an audience, not for three women in a fitting room on Avenue Marceau, and that changes the calculus.

The smoking still works. It works because Saint Laurent built it to last, and because Catroux showed him how a woman might actually wear it. That knowledge is in the pattern now, in the way the shoulder sits and the way the satin catches the light. You can buy it off the rail at Hedi Slimane prices, and it will fit. But it will not fit the way it did when the woman it was built for was standing in the room.