From Proto-Balto-Slavic, the later Balto-Slavic languages are thought to have developed, composed of the Baltic and Slavic sub-branches, and including modern Latvian, Lithuanian, Polish, Russian and Serbo-Croatian, among others.
There are several isoglosses that Baltic and Slavic languages share in phonology, morphology and accentology, which represent common innovations from Proto-Indo-European times and can be chronologically arranged.
[1] However, several new palatal (postalveolar) consonants had developed: *ś and *ź from earlier palatovelar plosives and *š from *s as a result of the Ruki sound law.
The modern interpretation, favoured by an increasing number of linguists, is that the acute was realised as glottalisation, an interruption of voicing similar to the stød found in Danish.
The most probable answer seems to be the assumption that PIE *s was changed to *š after *r, *u, *k, *i completely regularly within Balto-Slavic proper, but the traces of the effect of RUKI law were erased by subsequent changes in Lithuanian such as the change of word-final *-š to *-s. Generally, it can be ascertained that Lithuanian shows the effect of Ruki law only in old words inherited from Balto-Slavic period so Lithuanian š appears in words that have a complete formational and morphological correspondence in Slavic (ruling out the possibility of accidental, parallel formations).
The anomalous AP b neuters can be explained as remnants of suppletive patterns of PIE oxytone singulars and barytone collective plurals.
However, because it was blocked by an obstruent-final first syllable, a split occurred in this class of nouns: the retracted stems eventually joined AP c in Slavic, while the unretracted stems joined AP b. Olander's "mobility law"[16] presupposes some conditions, namely: The mobility law itself then states that the accent was deleted if it fell on the last mora of a word, and the word became inherently unaccented.
According to Jānis Endzelīns and Reinhold Trautmann *uR reflex resulted in zero-grade of morphemes that had PIE *o (> Balto-Slavic *a) in normal grade.
[clarification needed] Matasović, in 2008,[25] proposed the following rules: According to the traditional school, laryngeals disappeared as independent phonemes.
Frederik Kortlandt has proposed an alternative, more elegant and economic rule for the derivation of Balto-Slavic acute by using the glottalic theory framework of Proto-Indo-European.
In Slavic, in accordance with the "Law of Open Syllables", the final -n of the preposition was reinterpreted as belonging to the pronoun, which acted to preserve the nasal in its Balto-Slavic form, thus corroborating that it was indeed -n: If the change of *-m to *-n had not taken place at an earlier stage, the phrase would have been *śum eimiš, which would have given *sъ mimь in OCS instead.
The adjective are PIE *bʰardʰéh₂tos ("bearded") and its reflex in PSL *bordatъ (< PBS *bardā́ˀtas) and the PB *labas ("good"), declined in the masculine, feminine and neuter gender.
On the other hand, many dialects have completely lost tonal oppositions (such as some Kajkavian varieties, the Zagreb spoken nonstandard idiom).
A minority view, originating from Vladimir Dybo, considers Balto-Slavic accentuation (based on correspondences in the Germanic, Celtic, and Italic languages) more archaic than Greek-Vedic and therefore closer to Proto-Indo-European.
The classes of nominals are usually reconstructed on the basis of Vedic Sanskrit and Ancient Greek, which have retained the position of the original PIE accent almost unchanged.
For a long time, the exact relationships between the accentuation of nominals in Balto-Slavic and PIE was one of the most mysterious questions of Indo-European studies, and some parts of the puzzle are still missing.
Research conducted by Christian Stang, Ferdinand de Saussure, Vladislav Illich-Svitych and Vladimir Dybo has led to a conclusion that Balto-Slavic nominals, with regard to accentuation, could be reduced to two paradigms: fixed and mobile.
That theory has been criticized as leaving unclear why PIE nominals with fixed accent on the ending would become mobile, as analogies usually lead to uniformity and regularity.
De Saussure explained it as a result of accent retraction in the medially stressed syllables of consonant-stems exhibiting the hysterokinetic paradigm, with vocalic stems subsequently imitating the new accentual patterns by analogy.
In Proto-Slavic, the operation of Meillet's law converted acute roots to circumflexed in mobile nominals, so that the split found in Lithuanian does not occur.
For example, in Lithuanian, vowels /a/ and /e/ were lengthened when they initially bore short accent in open syllable, and rising tone emerged, marked with tilde sign ã.
There, the acute register is directly continued as a broken tone (lauztā) in originally unstressed syllables, marked with a circumflex diacritic: luôgs "window".
In originally-stressed syllables, the acute register is continued as a rising or lengthened intonation (stieptā), marked with a tilde: luõks "spring onion".
The circumflex register is generally continued as a falling intonation (krītošā), marked with a grave accent: lùoks "arch, bow".
Unlike the others, the broken tone can occur on all syllables: locative plural gal̂vâs "in the heads" (compare: Lithuanian galvosè with stress on a short final vowel, deleted in Latvian), including monosyllables: dêt "to lay eggs" < *dêtì.
In Czech, Slovak and Old Polish, the mobile accent was lost in favour of fixed stress, which rephonemicised the older accentual length distinctions.
A new rising accent (the "neoacute"), generally long, developed from retraction of the stress from a weak yer vowel (later usually lost).
The short rising accent that developed from the old acute (and in some circumstances, the neoacute) was later lengthened again in a number of Slavic languages (such as Russian, Czech, Slovenian).
In addition, the Proto-Slavic tonal distinction on liquid diphthongs is reflected fairly directly in Russian as a multisyllable accent shape (pleophony): *ôr (falling) > óro, *ór (rising) > oró.
Here is a table of basic accentual correspondences of the first syllable of a word: Scholars raised questions regarding the possible relationship between Slavic and Baltic languages as early as the late 18th century.