The Best Closet in New York By GABRIELLE BIRKNER Staff Reporter of the Sun Nan Kempner had the luxury of closet space. In her Park Avenue duplex, she had an ample wardrobe and dressing area of her own. As her three children grew up and moved out, she annexed their closets too allowing her to house an ever growing, rarely edited assortment of haute couture. By the time the legendary clotheshorse, society hostess, philanthropist, and wit died last year at the age of 74, she had amassed one of the world's largest couture collections. Those designs are the subject of " Nan Kempner: American Chic," an exhibit that opens next week at the Costume Institute of the Metropolitan Museum of Art. While the show will introduce Kempner's inimitable style to many museum visitors, the late style icon is already well known to those in New York's charitable nightlife scene, and its faithful observers. Kempner was renowned for her ability to appear simultaneously showy and tasteful and for the sheer volume of her fashion collection. She was not ambivalent about fashion, a New York society figure, Blaine Trump, said. "A lot of women say, Do I look good in this? Should I wear that?'" she said. "Not Nan. She had great confidence in her own eye and her own taste. She never questioned what was appropriate, or what others were going to be wearing. She set the standard." The Costume Institute show comprises five galleries, each devoted to a different aspect of Kempner's strategy its seasonlessness, its tailoring, its breadth, its most prevalent color scheme (black-and-white), and its white-tie glamour.