Another brand synonymous with trouser know-how is Theory, which launched in 1997. The brand still sells its original flat-front style, called the Max, which was particularly popular with working women. "There have been little adjustments like the belt width, but it was so clean, it didn't really age much," said Theory artistic director Olivier Theyskens, who joined the company in 2010. The label now offers a number of shapes, including several variations of cropped and straight cigarette pants. Mr. Theyskens said he's starting to revive the boot-cut style: "I think there's a new desire for them.