My first job as a teenage reporter on a local paper in 1969 had a dress code: no trousers. A man had to wear a tie and a woman wore a skirt.

Simple as that. My workplace rebellion came the day I turned up in a grey flannel Young Jaegertrouser suit (as worn by Jean Shrimpton and photographed by David Bailey), and was duly sent home.

As there was a time before the pill, so there was once life before the trouser suit, which Yves Saint Laurent, who died on Sunday, invented way-way back in 1966. Or rather he thought

a new thought: Le Smoking, the tuxedo for women that would become a permanent feature of his collections and would morph into the single most transformative piece of women’s wear since Chanel created the little black dress.

It was the perfect garment for the 70s and for women who went out to work on a daily basis. Women had been wearing trousers since the 20s, but pants had never managed to struggle out of the weekend and into the office.

The trousersuit put women on an equal sartorial footing with men. And the trousersuit, not the urban myth about bra-burning, is what fashion gave to feminism. When wearing it, your legs took longer steps; men looked at your face, not your ankles, and were forced to listen to

the words that came out of your mouth.

It killed the miniskirt stone dead. Hillary Clinton, a woman who does not possess good legs, has lived in trouser suits on the campaign trail for most of the November 2008 presidental election.

Yet, even when he dressed women in safari jackets and trenchcoats, Saint Laurent understood how to make them feel sexy. Le Smoking was not masculine but androgynous. At twenty-one, he had been anointed Dior’s successor on the death of the man who brought pleasure back to clothes after wartime rationing.

In the early 1960s, Brigitte Bardot declared that couture was for old ladies. Saint Laurent understood the next great change and the huge range of roles that women were about to play in the days ahead.

For well over two decades, Saint Laurent had his finger right on the button of the heady and hip times he lived in.