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 Frisian settlers on the coast of South Jutland (today's Northern Friesland) also spoke Old Frisian, but there are no known medieval texts from this area. The language of the earlier inhabitants of the region between the Zuiderzee and Ems River (the Frisians mentioned by Tacitus) is attested in only a few personal names and place-names. Old Frisian evolved into Middle Frisian, spoken from the 16th to the 19th century.

In the early Middle Ages, Frisia stretched from the area around Bruges, in what is now Belgium, to the Weser River in northern Germany . At the time, the Frisian language was spoken along the entire southern North Sea coast. This region is referred to as Greater Frisia or Magna Frisia, and many of the areas within it still treasure their Frisian heritage. However, by 1300, their territory had been pushed back to the Zuiderzee (now the IJsselmeer), and the Frisian language survives along the coast only as a substrate.

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.

Phonological development

Consonants
Generally, Old Frisian phonologically resembles Old English. In particular, it shares the palatalisation of velar consonants also found in Old English. For example, whereas the closely related Old Saxon and Old Dutch retain the velar in dag, Old Frisian has  and Old English has  . When initial and followed by front vowels the Germanic , changed to the sounds  and . Proto-Germanic  became  after , and word-initially before front vowels. Proto-Germanic , where it existed, became . The Old Frisian for church was  or , in Old English it was  , while Old Saxon and Old Dutch have the unpalatalised kirika. Palatalization postdated fronting, and predated monophthongization and i-umlaut.

Between vowels, h generally disappears ( from ), as in Old English and Old Dutch. Word-initial h- on the other hand is retained. Old Frisian retains th in all positions for longer than Old Dutch and Old Saxon do, showing the gradual spread of the shift from th to d from south to north, beginning in southern Germany in the 9th century, but not reaching Frisian until the 13th or 14th century.

Vowels
Another feature shared between the Old frisian and old english is Anglo-Frisian brightening, which fronted a to  except in certain conditions:
 stressed and before nasals
In the sequence (-)warC.
Before h(C), lC.
Much later, after breaking,  became .

Before , , , short ,  became  in a process known as "breaking". An unrelated sound change where  became  if  or  followed in the next syllable occurred later, after I-mutation.

Vowels were fronted or raised in before ,  a process called I-mutation:
 > 
 > 
 > 
 > 

The old Germanic diphthongs  and  become / and , respectively, in Old Frisian, as in / ("one") from Proto-Germanic , and  from  ("bread"). In comparison, these diphthongs become  and  ( and ) in Old English, and  and  ( and ) in Old Saxon. The diphthong  generally becomes , and Germanic  is retained. These diphthongs initially began with a syllabic (stressed) i, but the stress later shifts to the second component, giving to  and . For example,  ("people") and  from Proto-Germanic  and .

Phonology