The inhabitants of Skara Brae were makers and users of grooved ware, a distinctive style of pottery that had recently appeared in northern Scotland. The houses used earth sheltering, being sunk into the ground. They were sunk into mounds of pre-existing prehistoric domestic waste known as middens. This provided the houses with a stability and also acted as insulation against Orkney's harsh winter climate. On average, each house measures with a large square room containing a stone hearth used for heating and cooking. Given the number of homes, it seems likely that no more than fifty people lived in Skara Brae at any given time.