West Germanic gemination is a sound change that took place in all West Germanic languages

West Germanic languages

The West Germanic languages constitute the largest of the three traditional branches of the Germanic family of languages and include languages such as German, English, Dutch, Afrikaans, the Frisian languages, and Yiddish...

, around 300 AD. All single consonants except /r/ (original or from earlier /z/) were geminated

Gemination

In phonetics, gemination happens when a spoken consonant is pronounced for an audibly longer period of time than a short consonant. Gemination is distinct from stress and may appear independently of it....

 (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

Old Norse

Old Norse is a North Germanic language that was spoken by inhabitants of Scandinavia and inhabitants of their overseas settlements during the Viking Age, until about 1300....

, 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

Sievers' law

Sievers' law in Indo-European linguistics accounts for the pronunciation of a consonant cluster with a glide before a vowel as it was affected by the phonetics of the preceding syllable. Specifically it refers to the alternation between and , and possibly and , in Indo-European languages...

. Following the gemination, all word stems that ended in /j/ (except those ending in -r) were now heavy

Syllable weight

In linguistics, syllable weight is the concept that syllables pattern together according to the number and/or duration of segments in the rime. In classical poetry, both Greek and Latin, distinctions of syllable weight were fundamental to the meter of the line....

. 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

Germanic weak verb

In Germanic languages, including English, weak verbs are by far the largest group of verbs, which are therefore often regarded as the norm, though historically they are not the oldest or most original group.-General description:...

, 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

Old Saxon

Old Saxon, also known as Old Low German, is the earliest recorded form of Low German, documented from the 8th century until the 12th century, when it evolved into Middle Low German. It was spoken on the north-west coast of Germany and in the Netherlands by Saxon peoples...

 had dropped the /j/ in these words, but not before it /j/ had triggered i-Mutation

Germanic umlaut

In linguistics, umlaut is a process whereby a vowel is pronounced more like a following vowel or semivowel. The term umlaut was originally coined and is used principally in connection with the study of the Germanic languages...

, as well as palatalization

Palatalization

In linguistics, palatalization , also palatization, may refer to two different processes by which a sound, usually a consonant, comes to be produced with the tongue in a position in the mouth near the palate....

 in Old English

Old English language

Old English or Anglo-Saxon is an early form of the English language that was spoken and written by the Anglo-Saxons and their descendants in parts of what are now England and southeastern Scotland between at least the mid-5th century and the mid-12th century...

 and Old Frisian

Old Frisian

Old Frisian is a West Germanic language spoken between the 8th and 16th centuries in the area between the Rhine and Weser on the European North Sea coast. The Frisian settlers on the coast of South Jutland also spoke Old Frisian but no medieval texts of this area are known...

.

Examples:

Proto-Germanic	Gothic Gothic language Gothic is an extinct Germanic language that was spoken by the Goths. It is known primarily from the Codex Argenteus, a 6th-century copy of a 4th-century Bible translation, and is the only East Germanic language with a sizable Text corpus...	Old Norse	Old High German Old High German The term Old High German refers to the earliest stage of the German language and it conventionally covers the period from around 500 to 1050. Coherent written texts do not appear until the second half of the 8th century, and some treat the period before 750 as 'prehistoric' and date the start of...	Old Saxon Old Saxon Old Saxon, also known as Old Low German, is the earliest recorded form of Low German, documented from the 8th century until the 12th century, when it evolved into Middle Low German. It was spoken on the north-west coast of Germany and in the Netherlands by Saxon peoples...	Old English Old English language Old English or Anglo-Saxon is an early form of the English language that was spoken and written by the Anglo-Saxons and their descendants in parts of what are now England and southeastern Scotland between at least the mid-5th century and the mid-12th century...
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)