BY CHRISTINA BINKLEY

Kathy Beckett loves her go-to belt: a four-inch-wide black patent "cincher." The environmental attorney in Charleston, W. Va., admits it isn't comfortable: "Oh, heck no!" she says. "It's just for when I want to sit up straight all day."

Belts, for men, have a job—to tether their pants to their waists. And most men are familiar with their only serious belt rule: Match it with the shoes. For women, though, belts play a variety of character roles—as adornment, waist-defining corset, or finishing touch—and require an increasingly complex set of style decisions.

After a big push on the runways in the ...