Like other parts of the Hebrides, the island of Tiree had a significant number of Viking settlers in the Middle Ages. One of the apparent influences of their settlement is a building tradition known as the black house, a structure with very thick walls as in the picture at left. This design appears in far-flung parts of the North Atlantic region from Scandinavia to America. One reason for the wide diffusion of the black house is their insulation, keeping the interiors warm in winter and cool in summer.