The biggest differences in construction between YSL’s couture and RTW pants are in the fly zippers and the waistbands.

When I sew, I always look to the masters for inspiration. So when I plan on making trousers, I turn to Yves St. Laurent. According to YSL biographer Alex Madsen, St. Laurent believed that women—like men—should have a core wardrobe with a jacket and trousers; and, of course, he was just the designer to provide it. His tailored trousers were first introduced in his Fall/Winter 1966 couture collection and appeared soon thereafter in his Rive Gauche retail boutiques, and with that, a wardrobe basic was born. Of course, it didn’t hurt that French actress Catherine Deneuve bought a pair at the Paris Rive Gauche boutique on its opening day. (Remember that when pants first hit the high-fashion runways during the ’60s, most women had never worn dress pants in public; and ladies wearing pants were not permitted into better restaurants, at some well-known universities, or on military bases.)

So, what is it about this basic man-tailored trouser that has driven other designers and manufacturers to copy it in almost every fabric in all price ranges for more than three decades? And why does the tailored trouser continue to be the single most important pant design year after year? Simply because they are comfortable to wear and more flattering for more figures than any other pants style. Plus sewers love the fact that tailored trousers are relatively easy to sew. Let’s take a tour of these classics, zeroing in on the construction of the core features—the zippers and waistbands—in both the couture and retail versions.

Surface details

Through the years, the YSL basic trouser design has changed little. Some years the pants are wide, and…