West Germanic gemination is a sound change that took place in all West Germanic languages, around 300 AD. All single consonants except /r/ (original or from earlier /z/) were geminated (doubled) before /j/. The second element of the diphthongs iu and au was still underlyingly /w/ at this time and therefore was still considered a consonant, so it was geminated as well. Similar changes occurred in the history of Old Norse, but was much more limited, applying only to /k/ and /ɡ/.
The change affected only words with a short root vowel followed by a single consonant, as those with long vowels or with more than one consonant following the root vowel never contained /j/ after the ending consonant; /j/ had already been lengthened to /ij/ in these words through a rule known as Sievers' law. Following the gemination, all word stems that ended in /j/ (except those ending in -r) were now heavy. This set the stage for the later loss of /j/ following heavy syllables in the individual West Germanic languages.
This change particularly affected the infinitives of the first conjugation of weak verbs, which ended in /(i)jɑnɑ̃/, and also the short-stemmed ja(n)- and jō(n)-stem nouns and adjectives. By historical times (c. 800-900 AD), all of the West Germanic languages except Old Saxon had dropped the /j/ in these words, but not before it /j/ had triggered i-Mutation, as well as palatalization in Old English and Old Frisian.
||Old High German
||"to think" (gemination also in ON)
||hlæja (h lost in ON)
||"to turn" (no gemination before -ij-)
||"to heal" (no gemination before -ij-)
||"to carry" (no gemination of r)
||"to heal" (no gemination of r from z)