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 and West Germanic to Old English, Old High German and Middle Dutch. It is mainly in the dentals that these languages show significant differences in the patterns of grammatischer Wechsel. Note that this table lists only the outcome of word-internal consonants, since word-initial consonants were generally not affected by Verner's law.
| PIE | PG | ON | WG | OE | OHG | MDu | Notes |
|---|---|---|---|---|---|---|---|
| *p | *ɸ | f | *ɸ | f/v | f | f/v | By GL p→f. |
| *β | *β | b | By VL p→β. | ||||
| *t | *θ | ð | *θ | θ/ð | d | d | By GL t→θ. Then θ→d German and Dutch. |
| *ð | *d | d | t | By VL t→ð→d. Then d→t in German. | |||
| *k | *x | – | *x/h | x/- | x/h | x/- | By GL k→x. x→h before a vowel. h is then lost between vowels in Old English and Dutch. |
| *ɣ | ɣ | *ɣ | j/ɣ | ɡ | x/ɣ | By VL k→ɣ. Then ɣ→j in English and ɣ→ɡ in German, though all 3 use spelling ⟨g⟩. | |
| *kʷ | *xʷ | – | *x/h | x/- | x/h | x/- | Parallel to *k. |
| *ɣʷ | ɣ | *ɣ | j/ɣ | ɡ | x/ɣ | Parallel to *k, but *ɣʷ had split into *ɣ and *w by late Proto-Germanic. | |
| w/- | *w | w | w | w | |||
| *s | *s | s | *s | s/z | s | s/z | GL leaves s unaffected; allophone in English. |
| *z | r | *r | r | r | r | By VL s→z→r. |
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 grammatischer Wechsel 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.
Read more about this topic: Grammatischer Wechsel