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In historical linguistics, the High German consonant shift or second Germanic consonant shift is a phonological development (sound change) that took place in the southern parts of the West Germanic dialect continuum in several phases. It probably began between the third and fifth centuries and was almost complete before the earliest written records in High German were produced in the ninth century. The resulting language, Old High German, can be neatly contrasted with the other continental West Germanic languages, which for the most part did not experience the shift, and with Old English, which remained completely unaffected.

The High German consonant shift altered a number of consonants in the southern German dialects – and thus also in modern Standard German, Yiddish, and Luxembourgish – and so explains why many German words have different consonants from the obviously related words in English, Dutch and the Scandinavian languages. The term is sometimes used to refer to a core group of nine individual consonant modifications. Alternatively, it may encompass other phonological changes that took place in the same period. For the core group, there are three changes, which may be thought of as three successive phases. Each phase affected three consonants, making nine modifications in total:

Since phases 1 and 2 affect the same voiceless sounds, some scholars find it more convenient to treat them together, thus making for only a two-phase process: shifts in voiceless consonants (phases 1–2 of the three-phase model) and in voiced consonants (phase 3). The two-phase model has advantages for typology, but it does not reflect chronology.

Of the other changes that sometimes are bracketed within the High German consonant shift, the most important (sometimes thought of as the fourth phase) is:

This phenomenon is known as the High German consonant shift, because the core group affects the High German dialects in the mountainous south. It is also known as the "second Germanic" consonant shift to distinguish it from the "(first) Germanic consonant shift" as defined by Grimm's law and its refinement, Verner's law.

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