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).
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Sievers's law in the context of Indo-European sound laws
As the Proto-Indo-European language (PIE) broke up, its sound system diverged as well, as evidenced in various sound laws associated with the daughter Indo-European languages.Especially notable is the palatalization that produced the satem languages, along with the associated ruki sound law. Other notable changes include:
- Grimm's law and Verner's law in Proto-Germanic
- an independent change similar to Grimm's law in Armenian, dubbed the Armenian Consonant Shift
- Adjarian's law in Proto-Armenian
- loss of prevocalic *p- in Proto-Celtic
- Brugmann's law in Proto-Indo-Iranian
- Winter's law and Hirt's law in Balto-Slavic
- merging of voiced and breathy-voiced stops, and /a/ and /o/, in various "northern" languages
- Sturtevant's law
- Cowgill's laws of Germanic and Greek
- Osthoff's law
- Slavic first, second and third ("progressive") palatalization
Bartholomae's law in Indo-Iranian, and Sievers's law in Proto-Germanic and (to some extent) various other branches, may or may not have been common Indo-European features. A number of innovations, both phonological and morphological, represent areal features common to the Italic and Celtic languages; among them the development of labiovelars to labial consonants in some Italic and Celtic branches, producing "p-Celtic" and "q-Celtic" languages (likewise "p-Italic" and "q-Italic", although these terms are less used). Another grouping with many shared areal innovations comprises Greek, Indo-Iranian, and Armenian; among its common phonological innovations are Grassmann's law in Greek and Indo-Iranian, and weakening of pre-vocalic /s/ to /h/ in Greek, Iranian and Armenian.