In September 1978, Yves Saint Laurent threw a party for a perfume on a ship in New York harbour. The four-masted Peking was hung with coral-red paper lanterns, thousands of white cattleya orchids and a bronze Buddha that weighed 450 kilos, all to launch Opium in America. Opium had come out in Paris the year before. The house has never been shy about staging its fragrances.

YSL perfume is a slower story now, and the best example is Libre. Perfumer Anne Flipo started sketching it in 2011, and reaching the final formula reportedly took seven or eight years and about 1,570 attempts before it launched in 2019. She and Carlos Benaïm built it on French lavender, a note the brand says has traditionally belonged to men's perfumery, twisted with orange blossom from Morocco into what YSL calls an ultra-feminine version. It is a rare case of a fragrance's backstory actually explaining what is in the bottle.

The trick is to start with the original and let the flankers follow. The Libre Eau De Parfum with orange blossom and lavender is the base. The Intense adds warm vanilla, Le Parfum brings saffron, and the Vanille Couture edition goes further into vanilla with orange blossom. Berry Crush adds raspberry, and Flowers & Flames is the one I would smell last. The Eau de Toilette is the lightest way in, and the Libre Hair Mist is a soft finishing spray you can use without a full application.

Start with the coffee. Black Opium launched in 2014 as a rock'n'roll answer to the old Opium, and four perfumers built it, Nathalie Lorson, Marie Salamagne, Olivier Cresp and Honorine Blanc. They say it was the first time coffee had been used in such high doses in a feminine fragrance.

The Black Opium Eau de Parfum with coffee and white flowers is where I would begin, with pear, pink pepper and orange blossom on top, coffee, jasmine and bitter almond in the heart, and vanilla and patchouli below. Le Parfum leans into vanilla and coffee. The Glitter edition adds marshmallow, the Pink Glaze is the playful one, and Over Red brings in cherry.

Mon Paris arrived in 2016 from a different trio, Olivier Cresp, Harry Fremont and Dora Baghriche. It is a chypre fruity scent, with strawberry, raspberry and pear on top, jasmine sambac in the heart and patchouli underneath. Mon Paris Intensement adds white musk and rose to the frame. The Paris Eau de Toilette travel spray is the small one for a bag, and both lines are marketed by L'Oréal, which holds the license.

A pairing I like: Black Opium for a dinner in winter, and Mon Paris for the day. They sit at opposite ends of the same shelf.

What I would skip: a coffee and vanilla scent on a hot commute, where sweet notes can turn heavy fast.

One habit worth borrowing: spray a couture bottle on your skin and leave the house. Wait the afternoon out, then decide. These are quiet scents, and they need time.

Le Vestiaire des Parfums is YSL's smaller couture collection, and each bottle is named like a piece of clothing rather than a mood: Tuxedo, Coeur, Muse, Gold, Lavallière and Saharienne. It is a wardrobe you smell instead of wear. Coeur is built around precious orris butter, Gold around tuberose and ylang ylang, Lavallière around fig and rose, and Saharienne around neroli and musk. Muse and Tuxedo I would test side by side to see which mood you prefer.

Babycat is the first I would try, since bourbon vanilla and suede is the easiest entry point into the group. Libre Santal takes the Libre base and swaps in sandalwood for a warmer, woodier read of the same lavender and orange blossom idea.

None of these get the marketing push of the mainline bottles, which is part of why they feel like a quieter way to wear the house.

What I would skip: buying any of the couture bottles unsniffed. They are subtle enough that you cannot judge them from a description.

A habit worth borrowing: wear the mini for a full week before you decide. One afternoon lies. Seven days on a jacket collar and a wrist tell you what you actually have.

The Mini Black Opium & Libre Eau de Parfum Set is the obvious test for the two big names, and the Libre Collection Mini Perfume Duo Set is the same idea inside one family. Both are sensible ways to decide before a full bottle.

Travel sprays cover the rest. The Libre Eau de Parfum Intense Travel Spray, the Black Opium Travel Spray with coffee and vanilla and the Mon Paris Eau de Parfum Travel Spray are all sized for a bag, and I would keep one at work and one in the car.

The gift sets are the easy answer for someone else. The Libre Berry Crush Eau de Parfum Fragrance Gift Set and the Libre Eau de Parfum Travel Size Fragrance Gift Set are the two I would pick for a person who likes what they already wear.

The Libre Eau De Parfum Refill exists because the bottle's spray mechanism is built to be reloaded instead of thrown out. If you finish a bottle and know you love it, that is the tidy way to keep going.

I think Libre is the one to buy if you want a bottle that explains why this house is having a moment. Give it time to develop past the first ten minutes. If you want more attitude, Black Opium's coffee still reads as one of the more distinctive things on a counter of mostly vanilla and musk, and Mon Paris is what I would hand to someone who says they do not like sweet fragrances but secretly does.

Once you know which base you like, the couture shelf and the travel sizes are just ways to keep wearing it. YSL needed a ship and a bronze Buddha to launch a perfume. You need one bottle and a patient week. If you want to keep browsing, I also put together YSL Libre Perfume, the Lavender That Took Eight Years and YSL Libre Intense And The Libre Family.