Winter's law, named after Werner Winter, who postulated it in 1978, is a sound law operating on Balto-Slavic short vowels */e/, */o/, */a/ (< PIE *h₂e), */i/ and */u/, according to which they lengthen before unaspirated voiced stops in a closed syllable, and that syllable gains rising, acute accent.
- PIE *sed- "to sit" (that also gave Latin sedeō, Sanskrit sīdati, Ancient Greek hézomai and English sit) > Proto-Balto-Slavic *sēstej (*sēd-tej) > Lithuanian sė́sti, OCS sěsti (with regular *dt > *st dissimilation; OCS and Common Slavic yat /ě/ is a regular reflex of PIE/PBSl. */ē/).
- PIE *h₂ebl- "apple" (that also gave English apple) > Proto-Balto-Slavic *ābl- > standard Lithuanian obuolỹs (accusative óbuolį) and also dialectal forms of óbuolas and Samogitian óbulas, OCS ablъko, modern Serbian/Croatian jȁbuka, Slovene jábolko etc.
Winter's law is important for several reasons. Most importantly, it is supposed to show the difference between the reflexes of PIE */b/, */d/, */g/, */gʷ/ in Balto-Slavic (in front of which Winter's law operates in closed syllable), and PIE */bʰ/, */dʰ/, */gʰ/, */gʷʰ/ (before which there is no effect of Winter's law). This shows that 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.
Secondary, Winter's law is also supposed to show the difference between the reflexes of PIE *h₂e > */a/ and PIE */o/ which otherwise merged to */a/ in Balto-Slavic. When these vowels lengthen in accordance with Winter's law, one can see that old */a/ (< PIE *h₂e) has lengthened into Balto-Slavic */ā/ (which later gave Lithuanian /o/, Latvian /ā/, OCS /a/), and old */o/ has lengthened into Balto-Slavic */ō/ (which later gave Lithuanian and Latvian uo, but OCS /a/). In later development that represented Common Slavic innovation, the reflexes of Balto-Slavic */ā/ and */ō/ were merged, as one can see that they both result in OCS /a/. This also shows that Winter's law operated prior to the common Balto-Slavic change */o/ > */a/.
The original formulation of Winter's law stated that the vowels regularly lengthened in front of PIE voiced stops in all environments. As much as there were numerous examples that supported this formulation, there were also many counterexamples, such as OCS stogъ "stack" < PIE *stógos, OCS voda "water" < PIE *wodṓr (collective noun formed from PIE *wódr̥). An adjustment of Winter's law, with the conclusion that it operates only on closed syllables, was proposed by Matasović in 1994, which, unlike most of the other prior proposals, successfully explains away most counterexamples. Matasović's revision of Winter's law has been used in the Lexikon der indogermanischen Verben. Other variations of the blocking mechanism for Winter's law have been proposed by Kortlandt, Shintani, Rasmussen, Dybo and Holst but they have not gained wide acceptance.
Winter's law is not taken for granted by all specialists in Balto-Slavic historical linguistics. A study of counter-evidences led Patri (2006) to conclude that there is no law at all. According to him, exceptions to the law create a too heterogeneous and voluminous data-set to allow any phonological generalization.
See also 
- Lachmann's law – similar law occurring in Latin
- Kortlandt, Frederik (September 14-22 1988), "Remarks on Winter's law" (PDF), Studies in Slavic and General Linguistics: Dutch Contributions to the 10th International Congress of Slavists (Sofia) 11: 387–396
- Dybo, Vladimir (2002), "Balto-Slavic Accentology And Winter's Law" (PDF), Studia Linguarum (Moscow: RSUH Publishers) 3: 295–515, retrieved 2008-07-08
- Matasović, Ranko (2002), "Toward a relative chronology of the earliest Baltic and Slavic sound changes", Baltistica 40/2[dead link]
- Kapović, Mate (2008), Uvod u indoeuropsku lingvistiku (in Croatian), Zagreb: Matica hrvatska, ISBN 978-953-150-847-6
- Matasović, Ranko (2008), Poredbenopovijesna gramatika hrvatskoga jezika (in Croatian), Zagreb: Matica hrvatska, ISBN 978-953-150-840-7
- Patri, Sylvain (2005), "Observations sur la loi de Winter", Historische Sprachforschung (in French) 118: 269–293
- Young, Steven (2008), "Winter's law and etymologies, with special reference to Lithuanian" (PDF), Baltistica 43/2: 201–218