Winter's law

Winter's law, named after Werner Winter, who postulated it in 1978, is a proposed sound law operating on Balto-Slavic short vowels */e/, */o/, */a/, */i/ and */u/ according to which they lengthen before unaspirated voiced stops, and that syllable gains a rising, acute accent.
Compare:

PIE *sed- "to sit" > Proto-Balto-Slavic *sēˀstei > Lithuanian sė́sti, Old Church Slavonic sěsti.
PIE *h₂ébōl "apple" > Proto-Balto-Slavic *āˀbōl > standard Lithuanian and also dialectal forms of óbuolas and Samogitian óbulas, OCS ablъko, modern Serbo-Croatian, Slovene etc.

Winter's law distinguishes the Balto-Slavic reflexes of PIE */b/, */d/, */g/, */gʷ/ and PIE */bʰ/, */dʰ/, */gʰ/, */gʷʰ/. Therefore in relative chronology, Winter's law operated before PIE aspirated stops */bʰ/, */dʰ/, */gʰ/ merged with PIE plain voiced stops */b/, */d/, */g/ in Balto-Slavic.
Secondarily, it distinguishes the reflexes of PIE *h₂e > */a/ and PIE */o/ which otherwise merged to */a/ in Balto-Slavic. Winter's law lengthened old */a/ into Balto-Slavic */ā/ and old */o/ into Balto-Slavic */ō/. A later Common Slavic innovation merged the reflexes of Balto-Slavic */ā/ and */ō/ into OCS /a/, so Winter's law operated before the common Balto-Slavic change */o/ > */a/.
The original formulation claimed vowels regularly lengthened in front of PIE voiced stops in all environments. While numerous examples supported this, many counterexamples existed such as OCS stogъ "stack" < PBS *stagas < PIE *stógos, OCS voda "water" < PBS *wadō < PIE wodṓr. Matasović adjusted Winter's law in 1994 to operate only on closed syllables, which was used in the Lexikon der indogermanischen Verben. Kortlandt, Shintani, Rasmussen, Dybo and Holst vary the blocking mechanism differently.

Criticism

Not all Balto-Slavic historical linguists accept Winter's law. Patri found that exceptions to the law create a too heterogeneous and voluminous set of data to allow any phonological generalization into a law.