The distribution of the primary Germanic
dialect groups in Europe in around AD 1:
North Sea Germanic, or Ingvaeonic
Ingvaeonic /ˌɪŋviːˈɒnɪk/, also known as North Sea Germanic, is a postulated grouping of the West Germanic languages that comprises Old Frisian, Old English and Old Saxon.
Ingvaeonic is named after the Ingaevones, a West Germanic cultural group or proto-tribe along the North Sea coast. It is not thought of as a monolithic proto-language, but rather as a group of closely related dialects that underwent several areal changes in relative unison.
The grouping was first proposed in Nordgermanen und Alemanen (1942) by German linguist and philologist Friedrich Boster, as an alternative to the strict tree diagrams which had become popular following the work of 19th century linguist August Schleicher and which assumed the existence of a special Anglo-Frisian group. The other groupings are Istvaeonic, from the Istvaeones, including Dutch, Afrikaans, and related languages; and Irminonic, from the Irminones, including the High German languages.
Linguistic evidence for Ingvaeonic are common innovations observed in Old Frisian, Old English and Old Saxon such as the following:
- The so-called Ingvaeonic nasal spirant law, which (e.g.) converted *munþ "mouth" (cf. Old High German mund) into *mūþ (cf. Old English mūþ).
- The loss of the Germanic reflexive pronoun
- The reduction of the three Germanic verbal plural forms into one form ending in -þ
- The development of Class III weak verbs into a relic class consisting of four verbs (*sagjan "to say", *hugjan "to think", *habjan "to have", *libjan "to live")
- The split of the Class II weak verb ending *-ō- into *-ō-/-ōja-
- Development of a plural ending *-ōs in a-stem nouns (note, Gothic also has -ōs, but this is an independent development, caused by terminal devoicing of *-ōz)
- Possibly, the monophthongization of Germanic *ai to ē/ā, and *au to ō/ā (this may represent independent changes in Old Saxon and Anglo-Frisian)
Further reading 
- Bremmer, Rolf H. (2009). An Introduction to Old Frisian. Amsterdam: John Benjamins B.V. ISBN 978-90-272-3255-7.
- (German) Sonderegger, Stefan (1979). Grundzüge deutscher Sprachgeschichte. Diachronie des Sprachsystems. Band I: Einführung – Genealogie – Konstanten. Berlin/New York: Walter de Gruyter. ISBN 3-11-003570-7.
- Volyes, Joseph B. (1992). Early Germanic Grammar: Pre-, Proto-, and Post-Germanic. San Diego: Academic Press. ISBN 0-12-728270-X.
- Maurer, Friedrich (1942) Nordgermanen und Alemannen. Studien zur germanischen und frühdeutschen Sprachgeschichte, Stammes- und Volkskunde, Strasbourg: Hünenburg.