Some dresses have the power to make you feel instantly more elegant the moment you zip them up. Take Penelope: Cut from a heavyweight linen with a soft cotton lining, it has slim straps, a square neckline, and a beautifully fitted bodice. Below the waist, the long seams open into a full, sweeping skirt that falls to an ankle-grazing hem. It’s exactly what you want to wear for a candlelit dinner in Sag Harbor, an alfresco wedding on Shelter Island, or a midsummer gallery opening in East Hampton.