In historical linguistics, the German term grammatischer Wechsel ("grammatical alternation") refers to the effects of Verner's law when they are viewed synchronically within the paradigm of a Germanic verb.

According to Grimm's law, the Proto-Indo-European (PIE) voiceless stops *p, *t, *k and *kʷ usually became Proto-Germanic *ɸ, *θ (dental fricative), *x and *xʷ (velar fricative). Karl Verner identified the principle (Verner's law) that they instead become the voiced consonants *ƀ, *đ, *ǥ, *ǥʷ if they were immediately preceded within the same word by a syllable that was unaccented in Proto-Indo-European. (The Proto-Germanic phonemes *ƀ, *đ, *ǥ, *ǥʷ are thought to have had multiple phonetic realizations depending on the context (allophones), being phonetic voiced plosives [b d g gʷ] in some contexts, and voiced fricatives [β ð ɣ ɣʷ] in other contexts.) Furthermore, PIE *s, which usually came into Germanic unchanged, became *z in this position. This *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 those languages show significant differences in the patterns of grammatischer Wechsel. Note that the 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 Proto-Germanic voiceless fricatives *θ, *ɸ, and *s developed into voiced allophones, [ð], [v] and [z], when they were word-internal between voiced sounds, and in Middle Dutch also when word-initial; see: Pronunciation of English ⟨th⟩. In Old High German, Proto-Germanic *ƀ, *đ, *ǥ developed to /b, t, ɡ/ according to the High German consonant shift, and *θ became /d/. In Dutch, the idiosyncrasies of the 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, the outcomes of Proto-Germanic *φ and *ƀ merged in almost all Germanic languages (except Gothic and German), eliminating this variety early on. In Old Norse, *θ and *đ likewise merged altogether.

Grammatischer Wechsel is the phenomenon that a verb that in PIE had a stem ending in one of those 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 present in only primary underived verbs and so any derived verbs lacked perfect forms altogether. The latter verbs formed the base of the Germanic weak verbs and did not inherit the accent shift and so the alternation itself affects only Germanic strong verbs.

A process of levelling has meant that there are only a few examples in the modern languages. In East and North Germanic, the levelling was almost complete before the earliest records, but Gothic and Old Norse had traces of grammatischer Wechsel. In Old English, too, the levelling had already begun to the extent that in some verbs the preterite singular had taken the consonant of the preterite plural. The only surviving example in Modern English is was-were, but a trace can also be seen in the adjective forlorn, which reflects the old participle of the verb to lose, or sodden, which is originally a participle of seethe. This latter is parallelled by German sieden, sott, gesotten. German also features d-t in leiden, litt, gelitten ("to suffer") and schneiden, schnitt, geschnitten ("to cut"). One example of h:g is ziehen, zog, gezogen ("to pull"). All other cases have been levelled. Apart from the English copula mentioned above, the only occurrences of s-r in the modern languages are in Dutch: verliezen, verloor, verloren ("to lose") and verkiezen, verkoos, verkoren ("to choose").

Here are some examples:

NB. Not all consonant apophony in Germanic verbs is caused by grammatischer Wechsel. The consonant alternation in certain weak verbs that typically goes along with the Rückumlaut phenomenon (think:thought, German denken:dachte) is a result of a later development in Germanic known as the Germanic spirant law. Likewise, the terminal devoicing that produces a fortis-lenis alternation in Dutch (wrijven:wreef) is an unrelated historical phenomenon.