Here is the key to Chanel’s lasting success. The ageless (seventy-something, but who’s counting?) Karl Lagerfeld and the 18-year-old Emma Watson were photographed at yesterday’s Chanel show in Paris.

He designs for the house, she wears – and she is just one of many young women happy to wear Chanel, a label that has been going for seven decades.

Although not officially A Face of Chanel (yes, that is a real job) Watson, along with Lily Allen, Emma Thompson and Anita Pallenberg, enjoys a special relationship with the house, borrowing clothes and jewellery when she needs them and being flown in to sit in the front row at shows.

For a teenager she has already acquired an enviable array of outfits.

What the photograph also demonstrates is the power of a really good frock.

Watson, by being seen at fashion shows, is doing what many actresses do today – raising her public visibility. The fashion arena has become a powerful tool for celebrities to keep themselves in the public eye even when they don’t have a film or a record out.

For Watson it is also a useful tool for communicating to the public and to producers and directors that she is most definitely not the geeky Hogwarts student of the Harry Potter series, the character she has played since she was 11 years old.

While there’s a serious risk that any child actor, let alone one who has had the global success of Watson, becomes typecast, there is all the more need to show some kind of diversity, even if that diversity is confined to wearing a red dress one day and black PVC leggings the next.

There is one obvious link, however, between the costumes in Harry Potter and the clothes she wears from Chanel: at Hogwarts she may have been seen in school uniform, but by wearing Chanel she is endorsing a label whose style was born 80 years ago out of Coco Chanel’s desire to create a different kind of uniform for women.

The Chanel uniform was a relaxed, slouchy, deconstructed take on the clothes that Chanel herself borrowed from her aristocratic lovers’ wardrobe. At the time, it was groundbreaking and liberating. And the fact that a woman of Watson’s age is still wearing versions of that uniform is a testament to its legacy.

Those who are “faces” include Claudia Schiffer for ready-to-wear, Keira Knightley for Coco Mademoiselle, and Nicole Kidman for Chanel No 5, although after five years and a reputed fee of £5 million she is due to be replaced in January next year by Audrey Tautou.

After that, who knows? Perhaps one day it will be Emma Watson’s turn to be an official face.

Being a Chanel “face” is about as raunchy as it gets in advertising terms. Catherine Deneuve was one way back (as well as being a muse to Yves Saint Laurent. How’s that for scope?)

Deneuve never won an Oscar but she has a fabulous wardrobe.

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