Most designers work on the premise of 'planned obsolescence'. But when Yves Saint Laurent put women into dinner jackets and trousers in the autumn of 1966 with his first 'Le Smoking', he made planned obsolescence obsolete.

'So I am a classicist,' he said. 'I am fond of discipline . . . I've always wanted to give women protection from ridicule, freedom to be themselves.' And freedom, according to Saint Laurent, meant a trouser suit.

Of course, he was not the first to put women into pants. As far back as the Thirties, Elsa Schiaparelli had dressed women in lounging pyjamas. She went on to shock the Parisian bourgeoisie by declaring that trousers should be worn during the day. Coco Chanel, too, borrowed simple mannish shapes for her resort-wear, making trousers and loose cardigans in a fabric that had previously been used only for men's underwear: jersey.

But, maybe because of the complexities of his emotional life, the way he was drawn to anything exotic, beautiful or damned, Saint Laurent seemed better equipped than anyone to explore the tension between sexuality and role play. Against the increasing hedonism of the late Sixties and early Seventies, the image of Charlotte Rampling dressed in an Yves Saint Laurent 'smoking' and photographed by Helmut Newton took on a disturbing, edgy thrill.

Catherine Deneuve summed up the appeal of those first sharply cut evening suits. 'Saint Laurent designs for women with double lives,' she said. 'His clothes help a woman to confront the world of strangers. They permit her to go everywhere without drawing unwelcome attention . . . they give her a certain force, prepare her for encounters that may become a conflict of wills.'

This season, Yves Saint Laurent is not alone in questioning the need for frou-frou evening wear. Ralph Lauren, who has presented impeccable versions of menswear for women since the early Seventies, has done perhaps the most mannish dinner jacket and trousers, while Catherine Walker, for the Chelsea Design Company, has found room among the ballgowns for an Indian-inspired embroidered jacket and body.

None of these clothes are hard work it has to be the easiest thing in the world to shrug on a well-cut jacket and step into neatly tailored trousers. The discipline that we normally associate with men's dress the handful of basic shapes, the quiet luxury of good tailoring has been harnessed to give women freedom from awkward choices. There's no need for impact jewellery or a hairdo, no problems with shoes or tights. Instead, just push your feet into a pair of men's slippers.

Sadly, such freedoms don't come cheap. The Ralph Lauren evening jacket photographed here costs pounds 1,300; the shinier Armani suit a staggering pounds 13,000 (yes, that's thirteen thousand pounds but then, hand-beaded, one-off Armani creations are not known for being cheap). However, we have been assured that there are evening suits at Giorgio Armani for pounds 1,200. Those who expect their credit cards to work as hard as their wardrobe would be better off heading down to Jaeger, where there is a perfectly serviceable evening jacket for pounds 299.

(Photograph omitted)