When followed by, consonants were lengthened. The consonant, whether original or from earlier through rhotacization, was generally not affected; it occasionally shows gemination in Old High German, but inconsistently and this may be an analogical change. In contrast, the second element of the diphthongs iu and au was still underlyingly the consonant at this time, and therefore was lengthened as well. In Proto-Germanic, only appeared at the beginning of a syllable, primarily as the onset of a variety of suffixes and endings. It alternated with its syllabic counterpart in accordance with a phonological rule known as Sievers' law. This law states that consonantal appeared after a "light" syllable: one that contained a short vowel followed by at most one consonant. The syllabic allomorph appeared after "heavy" syllables, which included syllables containing a long vowel, a diphthong, or ending in more than one consonant. As the gemination itself required the consonant to be directly followed by, it therefore affected only light syllables; heavy syllables were not changed. Compare, for example, the Germanic verbs *fūlijaną "to defile" and *fuljaną "to fill, to make full", which appear in Old English as fȳlan and fyllan respectively, and in Old High German as fūlen and fullen; the first verb shows no gemination, while the second does. In the early history of most individual Germanic languages, syllabic was generally lost, while consonantal was retained. Earlier consonantal is also lost, however, after a consonant that underwent gemination. Thus, remains only after , while appears in all other cases, even those where Proto-Germanic had . It therefore appears that Sievers' law was still productive at this stage, and adapted to the new syllable length by changing the suffix from its consonantal to its syllabic variety.
Gemination triggered by /l/, /r/
West Germanic gemination also operated inconsistently on consonants followed by /l/ or /r/, e.g. Old English æppel "apple" < Proto-Germanic *aplaz. In some cases this led to doublets, e.g. West Saxon Old English tēar "tear " < *tæher < Proto-Germanic *tahraz vs. Northumbrian Old English tæhher "tear ".
Similar changes occurred in the history of Old Norse, although with a more limited scope. The change applied only to the combinations and, which were lengthened to and. Other consonants were not affected. Contrary to the changes in West Germanic, the remained nonsyllabic after the change, and was therefore retained rather than lost like syllabic was in other Old Norse words.
Effects
This change particularly affected the infinitives of the first conjugation of weak verbs, which ended in *-janą. It also affect the short-stemmed ja- and jō-stem nouns and adjectives. By historical times, all of the West Germanic languages except Old Saxon had lost medial syllabic, but not before any that it may have developed from had triggered i-Mutation. It also triggered palatalization of velar consonants in Old English and Old Frisian: and were geminated into palatal/postalveolar and, which then developed into geminate affricates and, spelled and in Old English. Examples:
