What constitutes cool these days? Alexa Chung, 28, working the music decks at a Chanel party? Charlotte Casiraghi, 25, the blue-blooded granddaughter of Grace Kelly, becoming the latest "face" of Gucci?
Agyness Deyn, 29, the chip shop worker-turned-model-turned-actress, who won rave reviews this week for her London West End stage debut?
Or Bip Ling, the teenage blogger/artist/model/DJ, who when not working, can be found helping out her father, William Ling, in his Mayfair art gallery, selling Andy Warhol prints?
These four aren't just cool girls, they are fashion's "it" girls. Not quite as famous as Rihanna but a league ahead of the "sidewalk catwalk" wannabes photographed by the street-style blogs Jak & Jil and Face Hunter.
Fashion it girls don't dress like the Vogue Japan editor Anna Dello Russo. Nor do they look like they've stepped out of the pages of her magazine. Yet they can influence what we wear and how we wear it as much as any trendsetting designer.
Casiraghi's kooky French-girl style helped make her the latest Gucci girl, in adverts photographed by Peter Lindbergh to celebrate the 90th anniversary of the Italian house.
Having spent years dodging modelling requests even from her fashion fairy godfather, Karl Lagerfeld (a friend of her mother, Princess Caroline of Monaco), she succumbed to Gucci, who sponsored her show-jumping career in 2010.
Unlike it girls of the past, who earned their title by fortunate birth into a powerful dynasty, the new generation (with the exception of the Monaco royal, obviously), don't always boast a listing in Debrett's or Who's Who.
It's more a combination of the fast-paced, jet-setting, multitasking nature of their glamorous yet hectic lives, coupled with the fact that in a world where everyone seems desperate to be told what to wear, fashion it girls do their own thing. That ultimately clinches them their title.
You can never quite predict what they will wear next. A Minnie Mouse bow in the hair, like the one Ling rocked at London Fashion Week. Adidas trainers with a glamorous evening frock, as worn by Deyn at the after-party following The Leisure Society's concert.
Thick grey tights peeping out of velvet platforms teamed with schoolgirlish grey and a namesake Mulberry bag, like Chung wore to New York Fashion Week. And a chic camel coat, belted, with a pair of Nike trainers and sunglasses propped up on her head, worn by Casiraghi recently. Expect to see it as a "look" in French Vogue any day. That and bushy eyebrows.
These girls don't even have to work at being fashion it girls, but they do have to work. Chung, 28, is a television presenter. Casiraghi is an amateur show jumper (and now model). Deyn is a model and now actress. Ling is a model and blogger. During London Fashion Week I spotted Ling interviewing front rowers, sketching and chatting at every show even during after-hours, while everyone else had sloped off home.
Stella McCartney, who arguably still qualifies for the it girl fashion title, recently spoke about wanting to instil the same work ethic in her children - one that she inherited from her own father, Sir Paul McCartney.
So does an it girl's "it" quality have something to do with our admiration of their work ethic, or the way they can still manage to look so cool despite having to cram so many events into their days?
The Gucci designer Frida Giannini has played down her momentous fashion coup, securing a third dynasty Monaco royal, saying she was impressed by Casiraghi's "modern attitude". At the end of the day it turns out the 25-year-old is just a regular family, fashion and horse-loving kinda girl. With a silk scarf tied nonchalantly around long, chestnut hair, a tight crimson hacking jacket and black jodhpurs, anyone not entirely sure who she is would want to find out. Immediately. Particularly anyone young who may not have noticed Gucci before. They will now. Watch out Burberry.
Giannini does have a point, though. There is realness to Casiraghi. Just as there is with Ling, Chung and Deyn, which comes across in the way they mix and match clothes, never doing matchy-matchy designer top-to-toe (which they could).
If only it was as easy as wearing designer labels. During the last round of fashion weeks, the American it girl Leigh Lezark, a DJ and model, appeared everywhere donned in designer clothes. But she failed to put her own spin on them. It was painful to watch.
The greatest myth in fashion is that you can buy style. Casiraghi has convinced me it's got to be in your genes.
Julia Robson is a London-based fashion journalist, broadcaster and stylist
