§ 140. In all WG languages, at an early stage of their independent history, most consonants were lengthened after a short vowel before [j]. This process is known as WG "gemination" or "doubling" of consonants, as the resulting long consonants are indicated by means of double letters, e. g.: *fuljan > OE fyllan (NE fill); *sætjan > OE settan (NE set), cf. Gt satjan.
During the process, or some time later, [j] was lost, so that the long consonants ceased to be phonetically conditioned. When the long and short consonants began to occur in identical phonetic conditions, namely between vowels, their distinction became phonemic.
The change did not affect the sonorant [r], e. g. OE werian (NE wear); nor did it operate if the consonant was preceded by a long vowel, e.g. OE dēmon, mētan. (NE deem, meet) — the earlier forms of these words contained [j], which had caused palatal mutation but had not led to the lengthening of consonants (the reconstruction of pre-written forms *mōtjan and *dōmjan is confirmed by OS motian and Gt domjan).