Sievers' Law

Sievers' 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, in Indo-European languages. For instance, Proto-Indo-European (PIE) *kor-yo-s became Gothic harjis "army", but PIE *ḱerdh-yo-s became Proto-Germanic *herdijas, Gothic hairdeis /herdĩs/ "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, a long vowel, or ending in more than one consonant), but *y would follow a light syllable (a short vowel followed by a single consonant).