The dress was in Yves Saint Laurent’s collection of Autumn-Winter, 1965. Harpers Bazaar called it “the assertive abstraction, a semaphore flag.” But what was the semaphore’s message? While Courrèges was cutting his space-age silhouette in peppy stripes and gingham, Saint Laurent’s “Mondrian” dress embraced modernity through art. Color-blocked to look like one of Piet Mondrian’s paintings, it was a masterful transformation of two dimensions into three—and also a sly way of saying that fashion, too, is an art. A new show examines a Saint Laurent masterpiece. —L.J.