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 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, 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).
This situation was first noticed by the Germanic philologist Eduard Sievers (1859-1932), and his aim was to account for certain phenomena in the Germanic languages. He originally discussed only *y in medial position. He also noted, almost as an aside, that something similar seemed to be going on in the earliest Sanskrit texts. Thus in the Rigveda dāivya- "divine" actually had three syllables in scansion (dāiviya-) but satya- "true" was scanned as written.
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