The Old Frisian reference article from the English Wikipedia on 24-Apr-2004
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Old Frisian

Old Frisian was the language spoken between the 8th and 16th century by the people who, from their ancient homes in North Germany and Denmark, had settled in the area between the Rhine and Elbe on the European North Sea coast in the 4th and 5th century. It is not known what language earlier inhabitants of the region, (the Frisians famously mentioned by Tacitus), used.

For most of that period Old Frisian was the official language of the region. After Friesland (now the Dutch province Friesland) lost its independence and became part of the Dutch Republic, Old Frisian lost that status.

There exist runic inscriptions in Old Frisian and from the 12th century untill 1550 official and legal documents survive.

Because Old Frisian was spoken by the same people who had settled in Britain, there is a close resemblance between Old Frisian and Old English.