How do West Germanic languages differ from North Germanic languages?

The West Germanic and North Germanic groups emerged from the fragmentation of the Northwest Germanic dialect continuum between the 3rd and 6th centuries CE. While they share a common ancestry, they are distinguished by specific phonological and grammatical developments.12

Key Linguistic Differences

• Consonant Gemination: In West Germanic, all consonants (except r) preceded by a short vowel and followed by j were geminated (doubled). In North Germanic, this process was restricted to velar consonants (k, g).1
• Final Consonants: West Germanic generally lost the Proto-Germanic final z in polysyllabic words, whereas North Germanic maintained it as r.1
• Vowel Changes: North Germanic underwent specific shifts, such as the rounding of unrounded vowels by a following u or w ("u-umlaut") and the diphthongization of short e to ja or jǫ. West Germanic is associated with the change of ð to d in all positions.21
• Grammatical Markers: North Germanic developed unique features, including the attachment of the reflexive pronoun to verbs to form a mediopassive and the use of the demonstrative inn as a definite article attached to nouns. West Germanic replaced inherited second-person singular past tense forms of strong verbs.21