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How closely related are English and Frisian?

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Thematic map of Europe displaying the distribution of Germanic languages, with different colors indicating regions where English, German, Netherlandic (Dutch), Frisian, Swedish, Danish, Norwegian, Icelandic, and Faroese are spoken. Striped areas mark regions where bilingualism is common, and language regions cover countries such as the United Kingdom, Germany, the Netherlands, Denmark, Sweden, Norway, Iceland, and parts of surrounding countries. — West Germanic languages Distribution of the Germanic languages in Europe.

Frisian is the West Germanic language most closely related to English. Both languages are descended from North Sea Germanic and share a unique linguistic history that distinguishes them from other Germanic languages.123

This close relationship is evidenced by several shared historical sound changes that occurred in both English and Frisian but not in other Germanic languages, such as German. These include:23

Loss of nasal sounds: The loss of nasal consonants before the Proto-Germanic voiceless fricatives f, þ, and s (e.g., five, mouth, and us).23
Palatalization: The palatalization of Proto-Germanic k and g before front vowels and j, which led to distinct developments in both languages (e.g., the ch sound in chin or the dg in bridge).23
Vowel shifts: Specific developments such as the fronting of the Proto-Germanic ē¹ and the backing and raising of nasalized ā (e.g., goose and other).23

Because of these shared features, linguists often group them together as Anglo-Frisian, a linguistic complex representing their common descent.4