Sievers's law in Indo-European linguistics accounts for the pronunciation of a consonant cluster with a glide ( or ) before a vowel as it was affected by the phonetics of the preceding syllable. Specifically it refers to the alternation between and , and possibly and as conditioned by the weight of the preceding syllable. For instance, Proto-Indo-European (PIE) became Proto-Germanic *harjaz, Gothic harjis "army", but PIE became Proto-Germanic *hirdijaz, Gothic hairdeis "shepherd". It differs from ablaut in that the alternation has no morphological relevance but is phonologically context-sensitive: PIE followed a heavy syllable (a syllable with a diphthong, a long vowel, or ending in more than one consonant), but would follow a light syllable (a short vowel followed by a single consonant).
History
Discovery
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 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.
Extension to other branches
After Sievers, scholars would find similar alternations in Greek and Latin, and alternation between and , though the evidence is poor for all of these. Through time, evidence was announced regarding similar alternations of syllabicity in the nasal and liquid consonants, though the evidence is extremely poor for these, despite the fact that such alternations would have left permanent, indeed irreversible, traces. For example, the Sanskrit "tool-suffix" -tra- (e.g. pā-tra- "drinking cup, vessel") almost always follows a consonant or long vowel and should have therefore been -tira-; but no such form as **pōtira-, either written as such or scanned thus, is actually attested in the Rigveda or any other Indic text. How a nearly universal suffix **-tira- would have been, or even could have been, uniformly replaced by -tra- is unobvious.
Edgerton
The most ambitious extension of Sievers's law was proposed by Franklin Edgerton (1885–1963) in a pair of articles in the journal Language ( and ). He argued that not only was the syllabicity of prevocalic consonants by context applicable to all six Indo-European sonorants (), it was applicable in all positions in the word. Thus a form like "sky" would have been pronounced like this only when it happened to follow a word ending with a short vowel. Everywhere else it would have had two syllables, . Edgerton also maintained that the phonotactic rules in question applied to sequences arising across morpheme boundaries, such as when the bahuvrīhi prefix occurred before a noun beginning with (e.g. "well-heroed", Vedic suvīra-). According to Edgerton, the word should have had two forms, depending on what immediately preceded it: and . This corollary he called the "converse" to Sievers's law, and is usually referred to as Edgerton's converse for short.
The evidence for alternation presented by Edgerton was of two sorts. He cited several hundred passages from the Rigveda, which he claimed should be rescanned to reveal hitherto unnoticed expressions of the syllable structure called for by his theory. But most forms show no such direct expressions; for them, Edgerton noted sharply skewed distributions that he interpreted as evidence for a lost alternation between syllabic and nonsyllabic consonants (commonly called "semivowels" in the literature). Thus say śiras "head" (from ) has no monosyllabic partner **śras (from ), but Edgerton noted that śiras occurred 100% of the time in the environments where his theory called for the syllabification of the . Appealing to the "formulaic" nature of oral poetry, especially in tricky and demanding literary forms like sacred Vedic versification, he reasoned that this was direct evidence for the previous existence of an alternant , on the assumption that when (for whatever reason) this *śras and other forms like it came to be shunned, the typical collocations in which they would have (correctly) occurred inevitably became obsolete at the same time as the loss of the form itself. And he was able to present a sizeable body of evidence in the form of these skewed distributions in both the 1934 and 1943 articles.
Edgerton's claims were immediately hailed by many in the scholarly community and enjoyed the status of orthodoxy among Indo-Europeanists for 35 or 40 years; in recent times they have not fared so well.