West Germanic gemination

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.

Examples:

Proto-Germanic	Gothic	Old Norse	Old High German	Old Saxon	Old English
hugjanaN	hugjan	hyggja	huggen	huggian	hycgan	"to think" (gemination also in ON)
bidjanaN	bidjan	biðja	bitten	biddian	biddan	"to ask"
hlahjanaN	hlahjan	hlæja (h lost in ON)	lahhan	hlahhian	hlæhhan	"to laugh"
wandijanaN	wandjan	venda	wenten	wendian	wendan	"to turn" (no gemination before -ij-)
hailijanaN	hailjan	heila	heilen	hēlian	hǣlan	"to heal" (no gemination before -ij-)
farjanaN	farjan	ferja	ferien	ferian	ferian	"to carry" (no gemination of r)
nazjanaN	nasjan	-	nerien	nerian	nerian	"to heal" (no gemination of r from z)