First found in Cheshire where they were anciently seated as Lords of the Manor. After the Battle of Hastings in 1066, William, Duke of Normandy, having prevailed over King Harold, granted most of Britain to his many victorious Barons. It was not uncommon to find a Baron, or a Bishop, with 60 or more Lordships scattered throughout the country. These he gave to his sons, nephews and other junior lines of his family and they became known as under-tenants. They adopted the Norman system of surnames which identified the under-tenant with his holdings so as to distinguish him from the senior stem of the family. After many rebellious wars between his Barons, Duke William, commissioned a census of all England to determine in 1086, settling once and for all, who held which land. He called the census the Domesday Book, indicating that those holders registered would hold the land until the end of time. Hence, conjecturally, the surname is descended from the tenant of the lands of Mere, and Norman noble named Mera, who held from Gilbert de Venables, originally of Venables near Evreux in Normandy, who was recorded in the Domesday Book census of 1086. Gilbert's main barony in Cheshire was at Kinderton, which he held from Hugh Lupus (The Wolf,) Earl of Chester.

Some of the first settlers of this family name or some of its variants were: Etienne Demers, who arrived in Quebec in 1644; Andre Demers, who arrived in Montreal in 1651; Thomas Demor, who arrived in Virginia in 1665; Ludowig Demer, who came to Pennsylvania in 1737.

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