In historical linguistics, the German term ("grammatical alternation") refers to the effects of Verner's law when they are viewed synchronically within the paradigm of a Germanic verb. Overview According to Grimm's law, the Proto-Indo-European (PIE) voiceless stops *p, *t, *k and *kʷ usually became Proto-Germanic *f, *θ (dental fricative), *x and *xʷ (velar fricative). Karl Verner identified the principle that these instead become the voiced consonants *b, *d, *g, *gʷ if they were word-internal and immediately preceded by an unaccented vowel in PIE. Furthermore, PIE *s, which usually came into Germanic unchanged, became *z in this position; Proto-Germanic *z later became North- and West Germanic *r. Consequently, five pairs of consonants emerged, each pair representing a single PIE phoneme. The following table shows the precise developments from Proto-Indo-European through Proto-Germanic to Old Norse, West Germanic, Old English, Old High German and Middle Dutch. It is mainly in the dentals that these languages show significant differences in the patterns of . Note that this table lists only the outcome of word-internal consonants, since word-initial consonants were generally not affected by Verner's law. In Old English, the fricatives took the voiced allophones , and when they were word-internal, and in Middle Dutch also when word-initial; see: Pronunciation of English ⟨th⟩. In Old High German, the stops were moved according to the High German consonant shift. In Dutch, the idiosyncrasies of this shift mean that Dutch (like German) experiences the shift þ→d but (like English) does not experience the shift d→t; thus the dental variety of is eliminated in Dutch by the normal operation of sound laws. Likewise, and merged in almost all Germanic languages (except Gothic and German), eliminating this variety early on. In Old Norse, and likewise merged altogether. Within verb paradigms is the phenomenon that a verb which in PIE had a stem ending in one of these phonemes displays a differing reflex in different parts of the paradigm, a result of the movable nature of accent in PIE. The Germanic past tense derives from the PIE perfect aspect, which was always athematic and therefore almost always had a shift of accent between the singular indicative (where it was on the root syllable) and the remaining forms including the past participle (where it was on the ending). However, the perfect aspect was only present in primary, underived verbs, and any derived verbs therefore lacked perfect forms altogether. These latter verbs