Decorating Duo Bernt Heiberg and William Cummings Deck out a Period Cottage in Sag Harbor

In the entryway, Flink, a 15-year-old Jack Russell terrier, sits on a Colonial-era chair alongside an antique Swedish side table.

The designers divided the adjacent living room into zones with a high-backed Heiberg Cummings sofa and a vintage farm table used as a desk; a prized Gustavian chair still has its original embroidery. Draperies throughout are fabricated from white Pindler & Pindler linen lined with classic gray wool flannel.

Two Heiberg Cummings chairs sit astride a pair of windows overlooking the front porch.

The original fireplace in the dining room stands out against contrasting walls painted Farrow & Ball’s Purbeck Stone. Twin settees face each other across a Scandinavian dropleaf table from Laurin Copen Antiques. The ceiling fixtures with Edison bulbs are from Rejuvenation ; the portrait above the mantel is an Edvard Munch drawing.

The serene, cozy master bedroom features a headboard from Restoration Hardware, an antique French settee, and a vintage American Windsor chair. Restoration Hardware

An outdoor shower on the second-floor deck stands just outside the master bath. The footed tub is from Blackman.

E. T. Raffel fabricated the cabinetry in the renovated kitchen; the subway tiles are from Nemo . The countertops are Carrara marble and the white porcelain farm sink is from Blackman.

The adjacent brick patio is original, although the wall of windows is new. The wicker chairs are from English Country Antiques.

To give the backyard a European touch, Heiberg and Cummings spread a layer of gravel in lieu of planting grass. A magnificent Norwegian pine shades a massive table from Hampton Teak. The planters are from Marders.