Sievers's law

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Sievers's law in Indo-European linguistics accounts for the pronunciation of a consonant cluster with a glide (*w or *y) before a vowel as it was affected by the phonetics of the preceding syllable. Specifically, it refers to the alternation between *iy and *y, and possibly *uw and *w as conditioned by the weight of the preceding syllable. For instance, Proto-Indo-European (PIE) *kor-yo-s became Proto-Germanic *harjaz, Gothic harjis "army", but PIE *ḱerdh-yo-s became Proto-Germanic *hirdijaz, Gothic hairdeis /hɛrdiːs/ "shepherd". It differs from ablaut in that the alternation has no morphological relevance but is phonologically context-sensitive: PIE *iy followed a heavy syllable (a syllable with a diphthong or long vowel, or ending in more than one consonant), but *y would follow a light syllable (a short vowel followed by a single consonant).