Danish Census Book
Adam of Bremen (ca. 1075) calls Vendsyssel Wendila, Ælnoth (ca. 1100) calls it Wendel, the Icelandic literature Vendill. Derived from this is the ethnic name wændlar, Danish vendelboer, which is part of the name of the "syssel". In the Danish Census Book (Kong Valdemars Jordebog, ca. 1231) Wændlesysæl, Wendelsysel, Wændil. Presumably originally the name of the Limfjord, then name of the region north of it.