Proto-Slavic, also Common Slavic, is the common ancestor of the Slavic languages, spoken around the 5th to 8th centuries AD. As with most other proto-languages, no attested writings have been found; the language has been reconstructed by applying the comparative method to all the attested Slavic languages as well as other Indo-European languages. However, a late form of this language as spoken in the region of Greek Macedonia, known as Old Church Slavonic, is attested from the 9th century AD.
The name "Proto-Slavic", by the definition of a proto-language, is the latest reconstructable common ancestor of all Slavic languages. Technically speaking, Proto-Slavic is purely a linguistic abstraction and admits no dialectal differentiation, since this would imply that a form was not ancestral to all of its descendants. When referring to the historical common language of the Slavs, the term Common Slavic is used, which generally includes a later stage than Proto-Slavic proper (up to the 10th century CE) where the language was dialectally differentiated yet still evolving in a unified fashion. During this period, the Slavic-speaking area expanded massively; yet sound changes still usually propagated throughout the entire area, although not always uniformly.
The ancestor of Proto-Slavic is Proto-Balto-Slavic, which is also the ancestor of the Baltic languages, e.g. Lithuanian and Latvian. This language in turn is descended from Proto-Indo-European, the parent language of the vast majority of European languages (including English, German, Spanish, French, etc.). Proto-Slavic gradually evolved into the various Slavic languages during the latter half of the first millennium CE, concurrent with the explosive growth of the Slavic-speaking area.
There is no scholarly consensus concerning either the number of stages involved in the development of the language (its periodization) or the terms used to describe them. For consistency and convenience, this article and the article on the history of the Slavic languages adopt the following scheme. See history of the Slavic languages for further discussion of the historical and linguistic development of Proto-Slavic from Proto-Balto-Slavic, and the further development of Proto-Slavic into the modern Slavic languages.
- Pre-Slavic (c. 1500 BCE — 300 CE): A long, stable period of gradual development. The most significant phonological developments during this period involved the prosodic system, e.g. tonal and other register distinctions on syllables.
- Early Common Slavic or simply Early Slavic (c. 300 — 600 CE): The early, uniform stage of Common Slavic, but also the beginning of a longer period of rapid phonological change. As there are no dialectal distinctions reconstructible from this period or earlier, this is the period for which a single common ancestor (that is, "Proto-Slavic proper") can be reconstructed.
- Middle Common Slavic (c. 600 — 800 CE): The stage with the earliest identifiable dialectal distinctions. Rapid phonological change continued, although with the massive expansion of the Slavic-speaking area. Although some dialectal variation did exist, most sound changes were still uniform and consistent in their application. By the end of this stage, the vowel and consonant phonemes of the language were largely the same as those still found in the modern languages. For this reason, reconstructed "Proto-Slavic" forms commonly found in scholarly works and etymological dictionaries normally correspond to this period.
- Late Common Slavic (c. 800 — 1000 CE, although perhaps through c. 1150 CE in Kievan Rus', in the far northeast): The last stage in which the whole Slavic-speaking area still functioned as a single language, with sound changes normally propagating throughout the entire area, although often with significant dialectal variation in the details.
This article considers primarily Middle Common Slavic, noting when there is slight dialectal variation. It also covers Late Common Slavic when there are significant developments that are shared (more or less) identically among all Slavic languages.
See Proto-Balto-Slavic language#Notation for much more detail on the uses of the most commonly encountered diacritics for indicating prosody (á, à, â, ã, ȁ, a̋, ā, ă) and various other phonetic distinctions (ą, ẹ, ė, š, ś, etc.) in different Balto-Slavic languages.
Vowel notation 
Two different and conflicting systems for denoting vowels are commonly in use in Indo-European and Balto-Slavic linguistics on one hand, and Slavic linguistics on the other. In the first, vowel length is consistently distinguished with a macron above the letter, while in the latter it is not clearly indicated. The following table explains these differences:
|Short front closed vowel (front yer)||i||ĭ or ь|
|Short back closed vowel (back yer)||u||ŭ or ъ|
|Short back open vowel||a||o|
|Long front closed vowel||ī||i|
|Long back closed vowel||ū||u|
|Long front open vowel (yat)||ē||ě|
|Long back open vowel||ā||a|
For consistency, all discussions of words in Early Slavic and before (the boundary corresponding roughly to the monophthongization of diphthongs, and the Slavic second palatalization) use the common Balto-Slavic notation of vowels. Discussions of Middle and Late Common Slavic, as well as later dialects, use the Slavic notation.
Other vowel and consonant diacritics 
- The haček on consonants (č ď ľ ň ř š ť ž) is used in this article to denote the consonants that result from iotation (coalescence with a /j/ that previously followed the consonant) and the Slavic first palatalization. This use is based on the Czech alphabet, and is shared by most Slavic languages and linguistic explanations about Slavic.
- The acute accent on the consonant ś indicates a special, more frontal "hissing" sound. The acute is used in several other Slavic languages (such as Polish, Serbo-Croatian and Macedonian) to denote a similar "frontal" quality to a consonant.
- The ogonek (ę ǫ), indicates vowel nasalization.
Prosodic notation 
For Middle and Late Common Slavic, the following marks are used to indicate tone and length distinctions on vowels, based on the standard notation in Serbo-Croatian:
- Acute accent ( á ): A long rising accent, originating from the Balto-Slavic "acute" accent. This occurred in the Middle Common Slavic period and earlier.
- Grave accent ( à ): A short rising accent. It occurred from Late Common Slavic onwards, and developed from the shortening of the original acute (long rising) tone.
- Inverted breve ( ȃ ): A long falling accent, originating from the Balto-Slavic "circumflex" accent. In Late Common Slavic, originally short (falling) vowels were lengthened in monosyllables under some circumstances, and are also written with this mark. This secondary circumflex occurs only on the original short vowels e, o, ь, ъ in an open syllable (i.e. when not forming part of a liquid diphthong).
- Double grave accent ( ȁ ): A short falling accent. It corresponds to the Balto-Slavic "short" accent. All short vowels that were not followed by a sonorant consonant originally carried this accent, until some were lengthened (see preceding item).
- Tilde ( ã ): Usually a long rising accent. This indicates the Late Common Slavic "neoacute" accent, which was usually long, but short when occurring on some syllables types in certain languages. It resulted from retraction of the accent (movement towards an earlier syllable) under certain circumstances, often when the Middle Common Slavic accent fell on a word-final final yer (*ь/ĭ or *ъ/ŭ).
- Macron ( ā ): A long vowel with no distinctive tone. In Middle Common Slavic, vowel length was an implicit part of the vowel (*e, *o, *ь, *ъ are inherently short, all others are inherently long), so this is usually redundant for Middle Common Slavic words. However, it became distinctive in Late Common Slavic after several shortenings and lengthenings had occurred.
Other prosodic diacritics 
There are unfortunately multiple competing systems used to indicate prosody in different Balto-Slavic languages (see Proto-Balto-Slavic language#Notation for more details). The most important for this article are:
- Three-way system of Proto-Slavic, Proto-Balto-Slavic, modern Lithuanian: Acute tone (á) vs. circumflex tone (ȃ or ã) vs. short accent (à).
- Four-way Serbo-Croatian system, also used in Slovenian and often in Slavic reconstructions: long rising (á), short rising (à), long falling (ȃ), short falling (ȁ). In the Chakavian dialect and other archaic dialects, the long rising accent is notated with a tilde (ã), indicating its normal origin in the Late Common Slavic neoacute accent (see above).
- Length only, as in Czech and Slovak: long (á) vs. short (a).
- Stress only, as in Russian, Ukrainian and Bulgarian: stressed (á) vs. unstressed (a).
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The following is an overview of the phonemes that are reconstructible for Middle Common Slavic.
Middle Common Slavic had the following vowel system:
On the front vowels:
- *e was open-mid [ɛ]. *ě was originally its longer counterpart, open-mid [ɛː], but was or became lowered to near-open [æː] in some dialects.
- *ь/ĭ was near-close [ɪ], *i was close [iː].
- The nasal vowel *ę was open-mid [ɛ̃ː]. It was later lowered to [æ̃ː] in several dialects.
The columns marked "central" and "back" may alternatively be interpreted as "back unrounded" and "back rounded" respectively, but rounding of back vowels was distinctive only between the vowels *y and *u. The other back vowels had optional non-distinctive rounding. Thus:
- *o was open [ɑ] in Early Slavic, later rounding to [ɒ], and then raising to [ɔ] towards the Late Common Slavic period. Its original long counterpart *a was open [ɑː] but did not acquire rounding.
- *ъ/ŭ was near-close [ʊ] or [ɯ̽]. Its original long counterpart *y was close back [ɯː] early on, with later fronting to [ɨː]. *u was [uː] with distinctive rounding.
- *ǫ was open-mid [ɔ̃ː], [ʌ̃ː] or close-mid [õː] or [ɤ̃ː]. This wide variety probably resulted from a late merger of phonemes, as earlier Slavic had two distinct back nasal vowels, corresponding to open *o and close *ъ/ŭ.
The vowels described as "short" and "long" were simultaneously distinguished by length and quality in Middle Common Slavic. Vowel length evolved as follows:
- In the Early Slavic period, length was the primary distinction (as indicated, for example, by Greek transcriptions of Slavic words, or early loanwords from Slavic into the Finnic languages).
- In the Middle Common Slavic period, all long/short vowel pairs also assumed distinct qualities, as indicated above.
- During the Late Common Slavic period, various lengthenings and shortenings occurred, creating new long counterparts of originally short vowels, and short counterparts of originally long vowels (e.g. long *o, short *a). The short close vowels *ь/ĭ and *ъ/ŭ were either lost or lowered to mid vowels, leaving the originally long high vowels *i, *y and *u with non-distinctive length. As a result, vowel quality became the primary distinction among the vowels, while length became conditioned by accent and other properties and was no a lexical property inherent in each vowel.
- Many modern Slavic languages have since lost all length distinctions.
Some authors avoid the terms "short" and "long", using "lax" and "tense" instead.
Middle Common Slavic had the following consonants:
The phonetic value (IPA symbol) of most consonants is the same as their traditional spelling. Some notes and exceptions:
- *c denotes a voiceless alveolar affricate [t͡s]. *dz was its voiced counterpart [d͡z].
- *š and *ž were postalveolar [ʃ] and [ʒ].
- *č and *dž were postalveolar affricates, [t͡ʃ] and [d͡ʒ], although the latter only occurred in the combination *ždž and had developed into *ž elsewhere.
- The pronunciation of *ť and *ď is not precisely known, though it is likely that they were held longer (geminate). They may have been palatalized dentals [tʲː dʲː], or perhaps true palatal [cː ɟː] as in modern Macedonian.
- *v was a labial approximant [ʋ]. It may have had bilabial [w] as an allophone in certain positions (as in modern Slovene).
- *l was [l]. Before back vowels, it was probably fairly strongly velarized [ɫ] in many dialects.
- The sonorants *ľ *ň were either palatalized [lʲ nʲ] or true palatal [ʎ ɲ].
- The pronunciation of *ř is not precisely known, but it was approximately a palatalized trill [rʲ]. It survives as a separate phoneme only in Czech, but also existed in earlier Polish (where it has since merged with *ž <ż>, but continues to be spelled <rz>). In other languages it either merged with *r, or dissimilated into *rj.
- *ś was [sʲ] or perhaps [ɕ]. It merged with *š in West Slavic, with *s in the other dialects.
A slight non-distinctive palatalization was probably present on all consonants that occurred before front vowels. When the high front yer *ь/ĭ was lost in many words, it left this palatalization as a "residue", which then became distinctive in most East and West Slavic languages.
Accent and tone 
As in its ancestors, Proto-Balto-Slavic and Proto-Indo-European, one syllable of each Common Slavic word was accented (carried more prominence). The placement of the accent was free and thus phonemic; it could occur on any syllable and its placement was inherently part of the word. The accent could also be either mobile or fixed, meaning that inflected forms of a word could have the accent on different syllables depending on the ending, or always on the same syllable.
Common Slavic vowels also had a pitch accent. In Middle Common Slavic, all accented long vowels, nasal vowels and liquid diphthongs had a distinction between two tones, traditionally called "acute" and "circumflex" accent. The acute accent was pronounced with rising intonation, while the circumflex accent had a falling intonation. Short vowels (*e *o *ь/ĭ ъ/ŭ) had no tonal distinction, and were always pronounced with falling intonation. Unaccented (unstressed) vowels never had tonal distinctions, but could still have length distinctions. These rules are similar to the restrictions that apply to the tones in Slovene.
In the Late Common Slavic period, several sound changes occurred. Long vowels bearing the acute (long rising) accent were usually shortened, resulting in a short rising intonation. Some short vowels were lengthened, creating new long falling vowels. A third type of pitch accent developed, known as the "neoacute", as a result of sound laws that retracted the accent (moved it to the preceding syllable). This occurred at a time when the Slavic-speaking area was already dialectally differentiated, and usually syllables with the acute and/or circumflex accent were shortened around the same time. Hence it is unclear whether there was ever a period in any dialect when there were three phonemically distinct pitch accents on long vowels. Nevertheless, taken together, these changes significantly altered the distribution of tones and vowel length, to the point that by the end of the Late Common Slavic period almost any vowel could be short or long, and almost any accented vowel could have falling or rising tone.
Most syllables in Middle Common Slavic were open. The only closed syllables were those that ended in a liquid (*l or *r), forming liquid diphthongs, and in such syllables, the preceding vowel had to be short. Consonant clusters were permitted, but only at the beginning of a syllable. Such a cluster and was syllabified with the cluster entirely in the following syllable, contrary to the syllabification rules that are known to apply to most languages. For example, *bogatĭstvo "wealth" was divided into syllables as *bo-ga-tĭ-stvo, with the whole cluster -stv- at the beginning of the syllable.
By the beginning of the Late Common Slavic period, all or nearly all syllables had become open as a result of developments in the liquid diphthongs. Syllables with liquid diphthongs beginning with an o or e had been converted into open syllables, e.g. *tort became *trot, *trat or *torot. The main exception are the Northern Lekhitic languages (Kashubian, extinct Slovincian and Polabian) only with lengthening of the syllable and no metathesis (*tart, e.g. PSl. *gord > Csb. gard; > Plb. *gard > gord). In West Slavic and South Slavic, liquid diphthongs beginning with ĭ or ŭ had likewise been converted into open syllables by converting the following liquid into a syllabic sonorant (palatal or non-palatal according to whether an ĭ or ŭ preceded). This left no closed syllables at all in these languages. The South Slavic languages, as well as Czech and Slovak, tended to preserve the syllabic sonorants, but in the Lekhitic languages (e.g. Polish), they fell apart again into vowel-consonant or consonant-vowel combinations. In East Slavic, the liquid diphthongs in ĭ or ŭ may have likewise become syllabic sonorants, but if so, the change was soon reversed, suggesting that it may never have happened in the first place.
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Proto-Slavic retained several of the grammatical categories inherited from Proto-Indo-European, especially in nominals (nouns and adjectives). Seven of the eight Indo-European cases had been retained (nominative, accusative, locative, genitive, dative, instrumental, vocative). The ablative had merged with the genitive. It also retained full use of the singular, dual and plural numbers, and still maintained a distinction between masculine, feminine and neuter gender. Verbs had become much more simplified, however, but displayed their own unique innovations.
As a result of the three palatalizations and the fronting of vowels before palatal consonants, both consonant and vowel alternations were frequent in paradigms, as well as in word derivation.
The following table lists various consonant alternations that occurred in Proto-Slavic, as a result of various suffixes or endings being attached to stems:
|+t (in infinitive)||t||t||t1||t2||st||st||st||st||t2||lt3||rt3||ť||ť||?||t1|
- ^1 Originally formed a diphthong with the preceding vowel, which then became a long monophthong.
- ^2 Forms a nasal vowel.
- ^3 Forms a liquid diphthong.
Vowels were fronted when following *j (which caused iotation) or a consonant that had been affected by the progressive palatalization. Because of this, most vowels occurred in pairs, labelled "soft" (after palatal consonants) and "hard" (elsewhere). The distinction between *ě₁ and *ě₂ is based on etymology and have different effects on a preceding consonant: *ě₁ triggers the first palatalization and then becomes *a, while *ě₂ triggers the second palatalization and does not change.
Most word stems therefore became classed as either "soft" or "hard", depending on whether their endings used soft (fronted) vowels or the original hard vowels. Hard stems displayed consonant alternations before endings with front vowels as a result of the two regressive palatalizations and iotation.
As part of its Indo-European heritage, Proto-Slavic also retained ablaut alternations, although these had been reduced to unproductive relics. The following table lists the combinations (vowel softening may alter the outcomes).
|zero grade||?||ь||ъ||ьl, ъl||ьr, ъr||ę, ǫ|
Although qualitative alternations (e-grade versus o-grade versus zero grade) were no longer productive, the Balto-Slavic languages had innovated a new kind of ablaut, in which length was the primary distinction. This created two new alternation patterns, which did not exist in PIE: short *e, *o, *ь, *ъ versus long *ě, *a, *i, *y. This type of alternation may have still been productive in Proto-Slavic, as a way to form imperfective verbs from perfective ones.
Most of the Proto-Indo-European declensional classes were retained. Some, such as u-stems and masculine i-stems, were gradually falling out of use and being replaced by other, more productive classes.
Adjective inflection had become more simplified compared to Proto-Indo-European. Only a single paradigm (in both hard and soft form) existed, descending from the PIE o- and a-stem inflection. I-stem and u-stem adjectives no longer existed. The present participle (from PIE *-nt-) still retained consonant stem endings.
Proto-Slavic had developed a distinction between "indefinite" and "definite" adjective inflection, much like Germanic strong and weak inflection. The definite inflection was used to refer to specific or known entities, similar to the use of the definite article "the" in English, while the indefinite inflection was unspecific or referred to unknown or arbitrary entities, like the English indefinite article "a". The indefinite inflection was identical to the inflection of o- and a-stem nouns, while the definite inflection was formed by suffixing the relative/anaphoric pronoun *jь to the end of the normal inflectional endings. Both the adjective and the suffixed pronoun were presumably declined as separate words originally, but already within Proto-Slavic they had become contracted and fused to some extent.
The Proto-Slavic system of verbal inflection was somewhat from the verbal system of Proto-Indo-European (PIE), although it was still rich in tenses, conjugations and verb-forming suffixes.
Grammatical categories 
The PIE mediopassive voice disappeared entirely except for the isolated form vědě "I know" in Old Church Slavonic (< Late PIE *woid-ai, a perfect mediopassive formation). However, a new analytic mediopassive was formed using the reflexive particle *sę, much as in the Romance languages. The imperative and subjunctive moods disappeared, while the old optative came to be used as the imperative instead.
In terms of PIE tense/aspect forms, the PIE imperfect was lost or merged with the PIE thematic aorist, and the PIE perfect was lost other than in the stem of the irregular verb *věděti "to know" (from PIE *woyd-). The aorist was retained, preserving the PIE thematic and sigmatic aorist types (the former is generally termed the root aorist in Slavic studies), and a new productive aorist arose from the sigmatic aorist by various analogical changes, e.g. replacing some of the original endings with thematic endings. (A similar development is observed in Greek and Sanskrit. In all three cases, the likely trigger was the phonological reduction of clusters like *-ss, *-st that arose when the original athematic endings were attached to the sigmatic *-s- affix.) A new synthetic imperfect was created by attaching a combination of the root and productive aorist endings to a stem suffix *-ěa- or *-aa-, of disputed origin. Various compound tenses were created, e.g. to express the future, conditional, perfect and pluperfect.
The three numbers (singular, dual and plural) were all maintained, as were the different athematic and thematic endings. (Only five athematic verbs exist: *věděti "to know", *byti "to be", *dati "to give", *ěsti "to eat" and *iměti "to have". dati has a finite stem *dad-, suggesting derivation by some sort of reduplication.) A new set of "semi-thematic" endings were formed by analogy (corresponding to modern conjugation class II), combining the thematic first singular ending with otherwise athematic endings. Proto-Slavic also maintained a large number of non-finite formations, including the infinitive, the supine, a verbal noun, and five participles (present active, present passive, past active, past passive and resultative). In large measure these directly continue PIE formations.
Proto-Indo-European had an extensive system of aspectual distinctions ("present" vs. "aorist" vs. "perfect" in traditional terminology), found throughout the system. Part of this was maintained in Proto-Slavic in the distinction between aorist and imperfect in the past tense. In addition, Proto-Slavic evolved a means of forming lexical aspect (verbs inherently marked with a particular aspect) using various prefixes and suffixes, which was eventually extended into a systematic means of specifying grammatical aspect using pairs of related lexical verbs, each with the same meaning as the other but inherently marked as either imperfective (denoting an ongoing action) or perfective (denoting a completed action). There are three primary ways in which the two verbs are related:
- A suffix is added to a more basic perfective verb to form an imperfective verb.
- A prefix is added to a more basic imperfective verb (possibly the output of the previous step) to form a perfective verb. Often, multiple perfective verbs can be formed this way using different prefixes, one of which echoes the basic meaning of the source verb while the others add various shades of meaning (cf. English "write" vs. "write down" vs. "write up" vs. "write out").
- The two verbs are suppletive — either based on two entirely different roots, or derived from different PIE verb classes of the same root, often with root-vowel changes going back to PIE ablaut formations.
In Proto-Slavic and Old Church Slavonic, the old and new aspect systems coexisted, but the new aspect has been gradually displacing the old one, and as a result most modern Slavic languages have lost the old imperfect, aorist, and most participles. A major exception, however, is Bulgarian (and also Macedonian to a fair extent), which has maintained both old and new systems and combined them to express fine shades of aspectual meaning. For example, in addition to imperfective imperfect forms and perfective aorist forms, Bulgarian can form a perfective imperfect (usually expressing a repeated series of completed actions considered subordinate to the "major" past actions) and an imperfective aorist (for "major" past events whose completion is not relevant to the narration).
Proto-Slavic also had paired motion verbs (e.g. "run", "walk", "swim", "fly", but also "ride", "carry", "lead", "chase", etc.). One of the pair expresses determinate action (motion to a specified place, e.g. "I walked to my friend's house") and the other expressing indeterminate action (motion to and then back, and motion without a specified goal). These pairs are generally related using either the suffixing or suppletive strategies of forming aspectual verbs. Each of the pair is also in fact a pair of perfective vs. imperfective verbs, where the perfective variant often uses a prefix *po-.
Many different PIE verb classes were retained in Proto-Slavic, including (among others) simple thematic presents, presents in *-n- and *-y-, stative verbs in *-ē- (cf. similar verbs in the Latin -ēre conjugation), factitive verbs in *-ā- (cf. the Latin -āre conjugation), and o-grade causatives in *-éye-.
The forms of each verb were based on two basic stems, one for the present and one for the infinitive/past. The present stem was used before endings beginning in a vowel, the infinitive/past stem before endings beginning in a consonant. In Old Church Slavonic grammars, verbs are traditionally divided into four (or five) conjugation classes, depending on the present stem. However, this division ignores the formation of the infinitive stem. The following table shows the main classes of verbs in Proto-Slavic, along with their traditional OCS conjugation classes. The "present" column shows the ending of the third person singular present.
|*nestì, *nesȅtь "carry, bring"
*bьrati, *beretь "take"
|PIE primary verbs, root ending in a consonant. Several irregular verbs, some showing ablaut. Not productive.|
|*bìti, *bь̏jetь "beat"
*dajati, *dajetь "give"
|PIE primary verbs, root ending in a vowel. -j- is inserted into the hiatus between root and ending. Several irregular verbs, some showing ablaut. Not productive.|
|-ajetь||-ati||*dělati, *dělajetь "do"||PIE denominatives in -eh₂-ye-. Remained very productive in Slavic.|
|-ějetь||-ěti||*uměti, *umějetь "know, be able"||PIE stative verbs in -eh₁-ye-. Somewhat productive.|
|*sъlàti, *sъljȅtь "send"
*cělovàti, *cělùjetь "greet, kiss"
|The basic type was marginally productive, and had iotation of the present stem. The subtype in -ovati was very productive and usually remains so in all Slavic languages.|
|2nd||-netь||-nǫti||*rìnǫti, *rìnetь "push, shove"||From various PIE n-suffix verbs, the nasal vowel was a Slavic innovation. Two subclasses existed: those with -nǫ- also in the aorist and participle, and those without.|
|4th||-itь||-iti||*prosìti, *prõsitь "ask"||PIE causative-iteratives in -éye-, denominatives in -eyé-. Remained very productive.|
|*mьněti, *mьnitь "think"
*slỳšati, *slỳšitь "hear"
|A relatively small class of stative verbs. The infinitive in -jati was a result of iotation, which triggered the change *jě > *ja.|
|5th||-(s)tь||-ti||*bỳti, *ȅstь "be"
*dàti, *dãstь "give"
*ě̀sti, *ě̃stь "eat"
*jьměti, *jьmatь "have"
*věděti, *věstь "know"
|PIE athematic verbs. Only five verbs, all irregular in one way or another.|
Accent classes 
In Late Common Slavic, there were three basic accent classes for nominals (nouns, adjectives, pronouns, participles):
- Class A, with a fixed acute accent on the stem (either on the root or on a morphological suffix).
- Class B, with largely fixed accent on the ending (on the first syllable of the ending, if multisyllabic).
- Class C ("mobile"), with alternation of the accent between the first syllable of the stem and the ending, depending on the paradigmatic form.
For this purpose, the "stem" includes any morphological suffixes (e.g. a diminutive suffix), but not generally on the inflectional suffix that indicates the word class (e.g. the -ā- of feminine ā-stem nouns), which is considered part of the ending.
Note that, because the accent in class A is acute, the accented stem syllable must be long (since the acute vs. circumflex distinction does not apply to short syllables). If the stem is multisyllabic, the accent can potentially fall on any stem syllable (e.g. *ję̄zū́k- "tongue"). In classes B and C, the stem syllable(s) can be either short or long. In class C, when the accent falls on the stem it is always non-acute (i.e. either long circumflex or short).
Some nouns (especially jā-stem nouns) fit into the class A pattern but have neoacute accent on the stem, which can have either a short or a long syllable. A standard example is *võľa "will", with neoacute accent on a short syllable. These nouns earlier belonged to class B; as a result, grammars may treat them as belonging either to classes A or B.
Originally in Balto-Slavic, there were only two accent classes, barytonic (with fixed stem accent) and mobile (with mobile accent), corresponding to Slavic classes A and C. Both classes originally had both acute and circumflex stems in them. Class B evolved during the Middle Common Slavic period from original class A nominals as a result of Dybo's law. This law moved the accent one syllable to the right, but only in class A nominals that did not have acute accent in the stem; this is why class A words consistently have acute accent. The lack of acute stems in Slavic class C is due to Meillet's law, which converted the acute to a circumflex accent in class C nominals.
Lithuanian developed along somewhat similar lines as a result of de Saussure's law, which split both the barytonic and mobile classes depending on whether the accented syllable of the stem was acute, producing the four modern Lithuanian noun classes: 1 (barytonic, acute); 2 (barytonic, non-acute); 3 (mobile, acute); 4 (mobile, non-acute). Words in Slavic classes A and B generally have cognates in Lithuanian classes 1 and 2, respectively, while words in Slavic class C have cognates in Lithuanian classes 3 and 4. The actual pattern of accent mobility (or lack thereof) also generally corresponds between Slavic classes A and C and Lithuanian classes 1 and 3, respectively — in both cases, these classes reflect the original Balto-Slavic barytonic and mobile patterns. However, the pattern of Slavic class B does not correspond to Lithuanian class 2 (which is mobile), because while Dybo's law and de Saussure's law both operated on the same sets of words, they produced different results. (In particular, de Saussure's law was restricted in that movement occurred only from a non-acute to an acute syllable.) See Proto-Balto-Slavic for more details.
During the Late Common Slavic period, the class B paradigm became mobile as a result of a complex series of changes that moved the accent leftward in certain circumstances, producing a neoacute accent on the newly stressed syllable. The paradigms below reflect these changes. All languages subsequently simplified the class B paradigms to varying degrees; the older situation can often only be seen in certain nouns in certain languages, or indirectly by way of features such as the Slovene neo-circumflex tone that carry echoes of the time when this tone developed. See History of the Slavic languages#Accentual developments for more details.
Verbs also had three accent classes (A, B and C) with similar characteristics to the corresponding noun classes. However, the situation is somewhat more complicated due to the large number of verb stem classes and the numerous forms in verbal paradigms.
Class A nouns 
|Masc. long -o||Neut. long -o||Masc. long -jo||Fem. long -ā||Fem. long -jā||Masc. long -i||Fem. long -i|
- The first form is the result in languages without contraction over /j/ (e.g. Russian), while the second form is the result in languages with such contraction. This contraction can occur only when both vowels flanking /j/ are unstressed, but when it occurs, it occurs fairly early in Late Common Slavic, before Dybo's law (the accentual shift leading to class B nouns). See below.
Note that all class A stems are long. This is because all such stems had Balto-Slavic acute register in the root, which can only occur on long syllables. (Short syllables, and long syllables with Balto-Slavic circumflex register, became class B nouns in Common Slavic.)
The distribution of short and long vowels in the stems without /j/ reflects the original vowel lengths, prior to the operation of Van Wijk's law, Dybo's law and Stang's law, which led to class B nouns and the differing lengths in /j/ stems.
Class B nouns 
|Masc. long -o||Neut. long -o||Masc. short -jo||Neut. short -jo||Fem. short -ā||Masc. long -i||Fem. short -i|
- The first form is the result in languages without contraction over /j/ (e.g. Russian), while the second form is the result in languages with such contraction. This contraction can occur only when both vowels flanking /j/ are unstressed, but when it occurs, it occurs before Dybo's law. At that point in this paradigm, stress was initial, allowing contraction to occur, resulting in a long *ī. As a result, after Dybo's law moved stress onto the vowel, it was retracted again by Stang's law. Without contraction, only Dybo's law applied.
Class B jā stem nouns are not listed here. The combination of Van Wijk's law and Stang's law would have originally produced a complex mobile paradigm in these nouns, different from the mobile paradigm of ā-stem and other nouns, but this was apparently simplified in Common Slavic times with a consistent neoacute accent on the stem, as if they were class A nouns. The class B jo stem nouns were also simplified, but less dramatically, with consistent ending stress in the singular but consistent root stress in the plural, as shown.
Class C nouns 
|Masc. short -o||Neut. long -o||Masc. long -jo||Neut. short -jo||Fem. short -ā||Fem. long -jā||Masc. long -i||Fem. short -i|
- The first form is the result in languages without contraction over /j/ (e.g. Russian), while the second form is the result in languages with such contraction. See the corresponding class A footnote.
- Verweij reconstructs i-stem genitive plural *zvěrь̃jь and *kostь̃jь, even though his reconstructed dative plural forms are *zvě̂rьmъ, *kȍstьmъ (see note below). This is because the strong yer preceding /j/ is a tense yer that is strong enough to block the supposed rule that skips intervening yers when retracting from a yer (see note below).
- These forms originally had final accent, which was retracted. Retraction from a yer skipped over intervening yers, even if strong. The result still should show neoacute accent, but according to Verweij, this is rarely found, and falling accent is the norm.
The accent pattern for the strong singular cases (nom., acc.) and all plural cases is straightforward:
- All weak cases (gen., dat., inst., loc.) in the plural are ending-stressed.
- The *-à ending that marks nom. sg. of the (j)ā-stems and nom./acc. pl. of the neuter (j)o-stems is ending-stressed.
- All other strong cases (sg. and pl.) are stem-stressed.
For the weak singular cases, it can be observed:
- All such cases in the (j)o-stems are stem-stressed.
- All such cases in the j(ā)- and i-stems are end-stressed except the dative. (However, the masculine i-stem inst. sg. is stem stressed because it is borrowed directly from the jo-stem.)
Note also that the long-rising vs. short-rising accent on ending-accented forms with Middle Common Slavic long vowels reflects original cirumflex vs. acute register, respectively.
|This section requires expansion. (February 2013)|
The same three classes occurred in verbs as well. Middle Common Slavic class B verbs in *-ī́tī had a neoacute retraction in Late Common Slavic in the present tense; that is, these verbs had original acute accent on the *-i- inflectional suffix in the infinitive, but neoacute accent on the stem in the present tense. This is due to the same process that caused neoacute retraction in class B jā-stem nouns (see above).
See also 
- History of the Slavic languages
- Old Church Slavonic
- Slavic liquid metathesis and pleophony
- Slavic languages
- Balto-Slavic languages
- Language family
- In English
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- In other languages
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