Ingvaeonic languages in the context of "Anglo-Frisian"

Old Frisian was a West Germanic language spoken between the late 13th century and the end of 16th century. It is the common ancestor of all the modern Frisian languages except for the Insular North Frisian dialects, with which Old Frisian shares a common ancestor called Pre–Old Frisian or Proto-Frisian. Old Frisian was spoken by contemporary Frisians who comprised a loose confederacy along the North Sea coast from around modern-day Bruges in Belgium to the Weser in modern-day northern Germany, dominating maritime trade. The vast majority of the surviving literature comprises legal documents and charters, though some poetry, historiographies, and religious documents are attested as well.

Old Frisian was closely related to and shared common characteristics with the forms of English and Low German spoken during the period. Although earlier scholarship contended that Frisian and English had a closer relationship to each other than to Low German, this is no longer the prevailing view. Old Frisian evolved into Middle Frisian around the turn of the 17th century, being largely pushed out by the emergence of Middle Low German as the language of trade in the North Sea. Scholars have argued that the term "Old Frisian" is somewhat misleading, since Old Frisian was contemporary with other Germanic languages during their "Middle" period, such as Middle English and Middle High German.

In historical linguistics, the Ingvaeonic nasal spirant law (also called the Anglo-Frisian or North Sea Germanic nasal spirant law) is a description of a phonological development that occurred in the Ingvaeonic dialects of the West Germanic languages. This includes Old English, Old Frisian, and Old Saxon, and to a lesser degree Old Dutch (Old Low Franconian).