By convention Frisia is divided into three sections. West Frisia, in the Netherlands, corresponds roughly to the Dutch province of Friesland - Fryslan in the local spelling - where the West Frisian language is still a co-official tongue spoken by hundreds of thousands of people. East Frisia, in the German state of Lower Saxony, covers the historic region around Aurich, Leer, Wittmund, and Emden, and in a broader sense the whole peninsula reaching into the North Sea. North Frisia, in Schleswig-Holstein, runs from the Eider River north to the Danish border, and includes the chain of North Frisian Islands where varieties of the North Frisian language are still spoken. Until the Second Schleswig War of 1864, the northern section belonged to the Danish Duchy of Schleswig. The Wadden Sea links all three regions along its southern shore - a single intertidal mudflat ecosystem that does for the modern Frisian identity what shared coast has always done: keeps the conversation going.

Long before the dike-builders changed the landscape, the Frisians lived on hills they made themselves. Pliny the Elder, writing in the first century, described the Frisii and their neighbors the Chauci as people who built up earthen mounds, called terps in the local language, to keep their farms above the storm surges of the North Sea. The settlements perched on these mounds were islands of human habitation in a periodically flooded plain, and many of the oldest Frisian villages still sit on the elevated rises that remember their terp foundations. The technology was simple, the engineering was ingenious, and the strategy worked for centuries before the medieval shift to a comprehensive dike system. The original Frisia comprised the present-day provinces of Friesland, Groningen, North Holland, and parts of South Holland - a coastal world that began at the level of the tide and worked up from there.

Here is a strange archaeological wrinkle. The Frisians of the Early Middle Ages were not, genetically, the same people as the Frisians of Roman times. The earlier population seems to have largely vanished from the coast in the centuries after Rome receded, and the Frisians who reappear in early medieval sources were largely the descendants of Anglo-Saxon immigrants who arrived from the German Bight during the Great Migration. They moved into the empty coastal niche, took on the old regional name, and carried Frisian-language culture along a coast that came to be called Greater Frisia - from northwestern Flanders all the way up to the Weser estuary, and later north along the Schleswig coast under Danish rule. Distant authors had trouble keeping the categories straight. The Byzantine writer Procopius listed three peoples of Great Britain: Angles, Frisians, and Britons. A Danish poet celebrating Canute the Great in the eleventh century used Frisian as a synonym for English.

Frisia survives as a self-conscious cultural region because the inhabitants have refused to let it lapse. The Interfrisian Council coordinates across the three modern subdivisions. Frisian-language broadcasting, schooling, and signage continue in each of the three areas, with different intensities. On the North Frisian Islands the words Frisia and Frisians often refer specifically to mainland North Frisia, not to the larger region. In Saterland Frisian, the German East Frisian language that survives in a single inland district, the word Fraeislound specifically means Ostfriesland. Every part of Frisia has its own local working definition of what Frisia means. The fact that the labels conflict is itself a sign that the identity is alive - that residents still care enough about the distinctions to keep them sharp.

From the air, Frisia is most legible at the tide line. The Wadden Sea exposes its mudflats twice a day along a continuous arc from the Dutch province of Friesland up through the East Frisian Islands and on toward the North Frisian Islands and Roemoe. Inland, the dike system traces the coast in a continuous green-and-grey ribbon, with sluice gates at every river mouth and polders ruled into rectangles behind. The differences between West, East, and North Frisia from cruising altitude are mainly differences in island shape - the long sandy spits of the East Frisian chain, the more fragmented North Frisian archipelago, the wide flat polders of the Dutch province. The shared signature is everywhere: a coast that has been rebuilt by its inhabitants, over centuries, with the patience of people who have never owned a country and have always owned the tide.