Old Frisian was a West Germanic language spoken between the 8th and 16th centuries along the North Sea coast, roughly between the mouths of the Rhine and Weser rivers.

The language of the earlier inhabitants of the region between the Zuiderzee and Ems River (the Frisii mentioned by Tacitus) is attested in only a few personal names and place-names.

In the early Middle Ages, Frisia stretched from the area around Bruges, in what is now Belgium, to the Weser River in northern Germany.

A close relationship exists between Old Frisian and Old English; this is due to a shared history, language and culture of the people from Northern Germany and Denmark who came to settle in England from around 400 A.D. onwards.

For example, whereas the closely related Old Saxon and Old Dutch retain the velar in dag, Old Frisian has dei and Old English has dæġ [dæj].

The Old Frisian for church was tzirke or tzerke, in Old English it was ċiriċe [ˈtʃiritʃe], while Old Saxon and Old Dutch have the unpalatalised kirika.

[7] Vowels were fronted or raised in before /i/, /j/ a process called I-mutation:[8] The old Germanic diphthongs *ai and *au become ē/ā and ā, respectively, in Old Frisian, as in ēn/ān ("one") from Proto-Germanic *ainaz, and brād from *braudą ("bread").

In comparison, these diphthongs become ā and ēa (ān and brēad) in Old English, and ē and ō (ēn and brōd) in Old Saxon.