Descriptions of late nineteenth-century Germanic dialects suggest or even explicitly claim that they were shaped in the Late Middle Ages or the Early Modern period. This implies that nineteenth-century dialects represented a language that had been nearly frozen for at least 300 years. The presumed stable character of the dialects was deliberately confirmed in the twentieth century by selecting old and conservative informants.This archaic perception of the nineteenth-century dialects is difficult to verify due to the lack of reliable dialectal data from the preceding centuries. West Frisian seems to be a sole exception to this. A moderate but fairly continuous flow of Frisian texts since ca. 1400, written in a language that did not develop a standardized form until the nineteenth century, opens a window on 600 years of language history. A series of ten case studies from various linguistic domains shows an ongoing dynamism over the centuries and no sign of early “frozen” dialects. The changes are either the result of language contact with Dutch or language internal pressure, and the geographical sources and directions of change reflect the shifting configurations in economy and demography through the centuries.