English church and West Frisian tsjerke, but Dutch kerk, Low German Kerk, Kark, and German Kirche
The early Anglo-Frisian and Old Saxon were spoken by intercommunicating populations, which led to shared linguistic traits through assimilation. English and Frisian have a proximal ancestral form in common before their divergence as geography isolated the settlers of the island from mainland Europe except contact with communities capable of open water navigation which resulted in Old Norse and Norman French influences on Modern English whereas Modern Frisian was subject to contact with the southernly Germanic populations restricted to the continent.
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 Alemannen (1942) by the German linguist and philologist Friedrich Maurer (1898–1984), as an alternative to the strict tree diagrams which had become popular following the work of the 19th-century linguist August Schleicher and which assumed the existence of an Anglo-Frisian group.
^Original meaning was "relative" which has become "brother or sister" in English.
^Hammarström, Harald; Forkel, Robert; Haspelmath, Martin, eds. (2017). "Anglo-Frisian". Glottolog 3.0. Jena, Germany: Max Planck Institute for the Science of Human History.
^Robert D. Fulk, “The Chronology of Anglo-Frisian Sound Changes”, Approaches to Old Frisian Philology, eds., Rolf H. Bremmer Jr., Thomas S.B. Johnston, and Oebele Vries (Amsterdam: Rodopoi, 1998), 185.
^Depending on dialect 1. en, jɪn, in, wan *e:, je: 2. twɑ:, twɔ:, twe:, twa: 3. θrəi, θri:, tri: 4. 'fʌu(ə)r, fuwr 5. fai:v, fɛv 6. saks 7. 'si:vən, 'se:vən, 'səivən 8. ext, ɛçt 9. nəin, nin 10. tɛn
^Grant, William; Dixon, James Main (1921) Manual of Modern Scots. Cambridge, University Press. p.105
Friedrich Maurer (1942), Nordgermanen und Alemannen: Studien zur Sprachgeschichte, Stammes- und Volkskunde, Strasbourg: Hünenburg.
Wolfram Euler (2013), Das Westgermanische [subtitle missing] (West Germanic: from its Emergence in the 3rd up until its Dissolution in the 7th Century CE: Analyses and Reconstruction). 244 p., in German with English summary, Verlag Inspiration Un Ltd., London/Berlin, ISBN978-3-9812110-7-8.