the Baltic and Slavic languages share a period of common development, the Proto-Balto-Slavic language. Common sound changes Winter's law: lengthening of vowels before Proto-Indo-European (PIE) non-breathy voiced consonants (*b, *d, *g). PIE breathy-voiced consonants (*bʰ, *dʰ, *gʰ, *ǵʰ) merge into plain voiced consonants (*b, *d, *g, *ǵ). This also occurred in several other Indo-European branches, but as Winter's law was sensitive to the difference between the two types of consonants, the merger must have happened after it and so is a specific Balto-Slavic innovation. Hirt's law: retraction of the PIE accent to the preceding syllable, if that syllable ended in a laryngeal (*h₁, *h₂, *h₃, see Laryngeal theory). A high vowel is inserted before PIE syllabic sonorants (*l̥, *r̥, *m̥, *n̥). This vowel is usually *i (giving *il, *ir, *im, *in) but in some occasions also *u (*ul, *ur, *um, *un). Proto-Germanic is the only other Indo-European language that inserts a high vowel (*u in all cases), all others insert mid or low vowels instead. Emergence of a register distinction on long syllables, between acute (probably glottalized) and circumflex. The acute arose primarily when the syllable ended in a PIE voiced consonant (as in Winter's law) or when it ended in a laryngeal. The distinction is reflected in most Balto-Slavic languages, including Proto-Slavic, as an opposition between rising and falling tone on accented syllables. Some Baltic languages directly reflect the acute register in the form of a so-called "broken tone". Shortening of vowels before word-final *m. Word-final *-mi > *-m after a long vowel. This followed the preceding change, as the preceding long vowel is retained. Raising of stressed *o to *u in a final syllable. Merging of PIE short *o and *a into *a. This change also occurred in several other Indo-European branches, but here too it must have happened after Winter's law: Winter's law lengthens *o to *ō and *a to *ā, and must therefore have occurred before the two sounds merged. It also followed the raising of *o to *u above. In the Slavic languages, *a is later rounded to *o, while the Baltic languages keep *a: Lithuanian ašìs Old Church Slavonic ось (from PIE *a: Latin axis, Ancient Greek áxōn) Lithuanian avìs, Old Church Slavonic овьца (from PIE *o: Latin ovis, Greec óis) Common Balto-Slavic innovations include several other changes, which are also shared by several other Indo-European branches. These are therefore not direct evidence for