West Friesland (also West Frisia; Dutch: West-Friesland; West Frisian language: West-Fryslân) is a historical region in the northern part of the Netherlands. It was located in parts of what now is Noord-Holland and the Waddenzee. The region was bordered by the rivers Vlie and IJ. Within this historical region is the contemporary region of West Friesland, which is a smaller area based on the Westfriese Omringdijk, a dyke system that lay in West Friesian district (gouw) of Westflinge.
The area between Vlie and IJ consists of the present-day municipalities of Alkmaar, Amsterdam (Landelijk Noord), Andijk, Anna Paulowna, Beemster, Bergen, Castricum, Den Helder, Drechterland, Edam-Volendam, Enkhuizen, Graft-De Rijp, Harenkarspel, Heerhugowaard, Heiloo, Hoorn, Landsmeer, Langedijk, Medemblik, Niedorp, Noorder-Koggenland, Purmerend, Obdam, Oostzaan, Opmeer, Schagen, Schermer, Stede Broec, Texel, Uitgeest, Vlieland, Waterland, Wervershoof, Wester-Koggenland, Wieringen, Wieringermeer, Wognum Wormerland, Zaanstad, Zeevang and Zijpe.
The river Vlie (or Fli), an extension of the IJssel branch of the Rhine, divided the northern Netherlands, which at the time was part of (West) Frisia, into a western and eastern part. In the 11th century, after heavy rainfalls, the river flooded and inundated large parts of the land. Not long after, the Zuider Zee bay (previously a lake) was formed, separating West Friesland from the contemporary Province of Friesland. In the Middle Ages, the Westflinge area of West Friesland practically became an island, bordered on the north by the Medem and Zijpe inlets, and to the south by various interconnecting lakes (now polder land) that were connected with the Zuider Zee. Because of this, the toponym "West Friesland" was applied more to the Westflinge area than the original West Friesland.
For about 300 years, West Friesland operated as an autonomous area as the West Frisians did not want to be subjected to authorities from Holland. Floris V, Count of Holland attempted to unite Holland and West Friesland during his reign, but it was his successor John I who finally defeated the West Frisians in 1297. However, even though West Friesland formed a united province with Holland in the Dutch Republic, it was recognized a separate region and the parliament of said province, commonly known as Holland, was formally known as the States of Holland and West Friesland, showing that West Friesland was still recognized in its own right. During the time of the United Provinces, West Friesland had its own independent Admiralty of the Northern Quarter and any admiral serving with that admiralty or the two other Hollandic admiralties, those of Amsterdam and the Maas (Rotterdam), had the title of Admiral of Holland and West Frisia.
The West Frisian language has disappeared from the region and the later West Frisian dialects are now slowly disappearing. Although these dialects are subdialects of Hollandic Dutch, they were strongly influenced in vocabulary and grammar by a West Frisian substratum.