Breathe in! It's the new must-have dress

It's not held together by safety pins, made of chiffon or encrusted with jewels, yet, as Rachel Weisz proved so elegantly this week, there's only one creation to be seen in this season. Susannah Frankel on the making of an 'it' dress

It was neither a little black dress nor a big white gown, both infinitely photogenic and familiar to the red carpet precisely for that reason. Any apparent understatement aside, however, Weisz's choice of garment was the latest in a long line of "it" dresses that include everything from "that" sheath, held together by safety pins, worn by Elizabeth Hurley almost 10 years ago and designed by Gianni Versace, to the Chloé dress sported by Kylie Minogue, copies of which pretty much kept the British high street in business throughout that season and into the next.

A dress becomes an "it" dress for many reasons. It may be the actress that makes the outfit (Sienna Miller in Matthew Williamson) or vice versa (Versace and Hurley again). The best kind of "it" dress taps into the zeitgeist. Sarah Jessica Parker and Nicole Kidman in New York earlier this summer in an ocean of girlish Yves Saint Laurent polka dots and frills perfectly suited the blithe optimism of that particular fashion moment as did Charlotte Gainsbourg and Jennifer Connolly, equally radiant in ultra-feminine feathered Balenciaga.

Just about anyone you'd care to mention wearing Lanvin has seemed equally timely for the past few, vintage fashion-obsessed years. Kate Moss preferred hers jewelled; Natalie Portman favoured pleats, Fortuny style. An "it" dress may suit the occasion it is worn for. Madonna, proud to be prim and proper, bespectacled in rose-printed Prada at her 2003 book launch is one prime example. Failing that, it may simply be supremely flattering. Think Sharon Stone in liquid gold Christian Dior at this year's Cannes Film Festival or Kidman in French navy Jean Paul Gaultier haute couture at the Oscars the year before.

Weisz's dress was designed by the Frenchman, Roland Mouret. It has "it" status in spades which is presumably why, in just six months, Demi Moore, Naomi Watts, Keira Knightley and Kidman have all been photographed in remarkably similar attire. In fact, Mouret launched the silhouette early this year and first sent it into the spotlight on Scarlett Johansson at the Oscars. Given an unprecedented following such as this one - it is very rare that an actress will deign to wear something seen on the back of one of her peers - it's small wonder that Mouret re-invented the dress for the forthcoming season also. Rachel Weisz wore her particular "it" dress this week. The rest of the world will have to wait until January for this version - it's slightly longer than last season's, which came, most famously, in Prince of Wales check - and will, no doubt, be more than happy to pay £850 for the privilege of owning it.

It must surely be all part of the equation that Mouret has the personal credentials to back up the frock: he is colourful, handsome charismatic, and is master of the sharply controversial sound bite. He once said: "Everything in my life has always been linked to dresses. My dad was a butcher, so he wore a long apron." He was born in Lourdes 44 years ago. While his father supplied the local population with fine cuts of meat, his mother worked as a waitress. "It was like growing up in a funfair. All the lights in the shops. It was like a religious Dallas. At first I wanted to be a priest but when I was 13 I told them at school that I wanted to be a fashion designer. The headmaster called my dad and said, 'you know, he can't do that'." Throughout the Eighties, Mouret worked in Paris. He went briefly to fashion school but decided "I'd rather go out, have a life. I was a club kid, my look was quite Forties, quite Dick Tracy. I made my own clothes during the day and bought more at the flea market."

Coming, like John Galliano, from the Taboo generation and therefore of the mindset that one ought to stand out in a crowd, he worked as a model, as a stylist (he once designed a pattern for the readers of French Elle for a dress that would cost them the grand sum of £5 to make) and as an art director. In 1994, having moved to London, he opened the fashionable Soho bar Freedom. "I met Lee [Alexander] McQueen there, and Katy [England] and Hussein [Chalayan]."

In February 1998 he launched his first collection under his own name at London Fashion Week. A heady mix of Hollywood glamour, fused with Parisian chic and Madame Grès-style drape and all with a fashionably raw edge, it was well received critically but, as with so many designers who set up in this country, commercial success was less readily forthcoming. Four years ago now, however, with new backing, Mouret hit the international scene. In 2003 he began showing his collection alongside some of the world's major players in New York - he has not looked back.

It was not until Mouret came up with the dress in question here, however, that's his appeal really went global. There are now whole spreads devoted to the designer in the hugely powerful US edition of In-Style magazine. Any celebrity worth their fashion credentials, meanwhile, has at least one Roland Mouret outfit to her name.

If, at first sighting, Mouret's "it" dress seems low-key, one need only turn it inside out to discover that it is, in fact, rather more complicated. Heralding the return of the more structured garment after seasons of whimsical, bohemian froth it is mounted (a formidable concept in this day and age) on what those in the know describe as "paramesh", an (also formidable) highly elastic techno fabric originally used to reinforce 1950s underwear. If the hefty foundation garments of that era are today far from de rigueur, the hour-glass shape that they achieved is, conversely, seen as the height of good taste once more.

For those who embrace the look to the full, Mouret's dress comes with a "waspie" - basically, a corset - for added support. "It's a two-fold attack. We thought it was about time to redefine the body," a spokesperson for the company confirmed. And A-listers the world over clearly agree. In the end, the attraction of Weisz's dress is that it looks ultra-chic and, in its slight asymmetry, perfectly modern but harks back to a time when Jack Lemmon marvelled at Marilyn Monroe's ability to actually move about in her figure-hugging clothes in Some Like It Hot. Jayne Mansfield in The Girl Can't Help It and any number of Hitchcock's heroines were, of course, all also in possession of equally impressive, undulating curves.

Mouret has said: "Some designers dress the woman inside them. I want to dress the woman at my side." Suffice it to say Mouret's woman references the proverbial arm candy of eras gone by but is a far more knowing creature than her predecessor. Is it any wonder, then, that she seems so appealing just now?