The phonology of the Proto-Indo-European language (PIE) has been reconstructed by linguists, based on the similarities and differences among current and extinct Indo-European languages. Because PIE was not written, linguists must rely on the evidence of its earliest attested descendants, such as Hittite, Sanskrit, Ancient Greek, and Latin, to reconstruct its phonology.

The reconstruction of abstract units of PIE phonological systems (i.e. segments, or phonemes in traditional phonology) is mostly uncontroversial, although areas of dispute remain. Their phonetic interpretation is harder to establish; this pertains especially to the vowels, the so-called laryngeals, the palatal and plain velars and the voiced and voiced aspirated stops.

Phonemic inventory Proto-Indo-European is reconstructed as having the following phonemes. Note that the phonemes are marked with asterisks to show that they are from a reconstructed language. See the article on Indo-European sound laws for a summary of how these phonemes reflected in the various Indo-European languages.

Consonants Labial CoronalDorsal Laryngeal palatal velar lab. vel. Nasal Stop voiceless voiced () aspirated Fricative Sonorant

The table uses the Wiktionary's notation for transcribing Proto-Indo-European; variant transcriptions often seen elsewhere are provided for individual segments in the following sections. Superscript stands for aspiration, and superscript for labialization. The consonant is the palatal semivowel (whose IPA transcription is and not ).

Stop series Proto-Indo-European was formerly reconstructed with four series of stops: voiceless unaspirated, voiceless aspirated, voiced unaspirated, and voiced aspirated (such as *t, *tʰ, *d, *dʰ). More recent reconstructions analyze voiceless aspirated stops as sequences of stop and laryngeal, and so the standard reconstruction now includes only three series of stops, with the traditional phonetic descriptions of voiceless, voiced and voiced aspirated. (Sanskrit has a fourfold distinction, including a voiceless aspirated series), and it is typologically rare across attested languages. The absence or rarity of *b (see below) is also unusual. Additionally, Proto-Indo-European roots have a constraint that forbids roots from mixing voiceless and voiced aspirate stops or from containing two voiced stops. These considerations have led some scholars to propose a glottalic theory of the PIE stop system, replacing the voiced stops with glottalized and the voiced aspirated stops with plain voiced. Direct evidence for glottalization is limited, but there is some indirect evidence, including Winter's law in Balto-Slavic.

Labials and coronals

PIE are grouped with the cover symbol P. The phonemic status of is disputed: it seems not to appear as an initial consonant (except in a few dubious roots such as *bel-, noted below), while reconstructed roots with internal *b are usually restricted to Western branches, casting doubt on their validity for PIE.Tomic, O.M., Markedness in Synchrony and Diachrony, de Gruyter 1989, p. 99.

Some have attempted to explain away the few roots with *b as a result of later phonological developments.See for example Ringe, D.A., On the Chronology of Sound Changes in Tocharian, AOS 1996, p. 152. Suggested such developments include *ml- > *bl-, connecting the dubious root 'power, strength' (> Sanskrit bálam, Ancient Greek ) with mel- in Latin , and *h₂ebl-/ 'apple' with a hypothetical earlier form *h₂eml-, which is in unmetathesized form attested in another reconstructible PIE word for apple, *méh₂lom (> Hittite maḫla-, Latin , Ancient Greek ). In PIE *ph₃ the *p regularly gives *b; for example, the reduplicated present stem of 'to drink' > *pi-ph₃- > Sanskrit píbati.

At best, PIE remains a highly marginal phoneme.

The standard reconstruction identifies three coronal, or dental, stops: . They are symbolically grouped with the cover symbol T.

Dorsals

According to the traditional reconstruction, such as the one laid out in Brugmann's Grundriß der vergleichenden Grammatik der indogermanischen Sprachen more than a century ago, three series of velars are reconstructed for PIE: "Palatovelars" (or simply "palatals"), (also transcribed or or . "Plain velars" (or "pure velars"), . Labiovelars, (also transcribed ). The superscript or stands for labialization (lip rounding) accompanying the velar articulation.

The actual pronunciation of these sounds in PIE is not certain. One current idea is that the "palatovelars" were in fact simple velars, i.e. , while the "plain velars" were pronounced farther back, perhaps as uvular consonants, i.e. .Kümmel, M.J. (2007), Konsonantenwandel. Bausteine zu einer Typologie des Lautwandels und ihre Konsequenzen für die vergleichende Rekonstruktion. Wiesbaden: Reichert. Cited in Prescott, C., Pharyngealization and the three dorsal stop series of Proto-Indo-European. If the labiovelars were just labialized forms of the "plain velars", they would then have been pronounced but the pronunciation of the labiovelars as would still be possible in uvular theory, if the satem languages first shifted the "palatovelars" and then later merged the "plain velars" and "labiovelars". See for more support of this theory.

Another theory is that there may have been only two series (plain velar and labiovelar) in PIE, with the palatalized velars arising originally as a conditioned sound change in satem languages. See .

The satem languages merged the labiovelars with the plain velar series , while the palatovelars became sibilant fricatives or affricates of various types, depending on the individual language. In some phonological conditions, depalatalization occurred, yielding what appears to be a centum reflex in a satem language. For example, in Balto-Slavic and Albanian, palatovelars were depalatalized before resonants unless the latter were followed by a front vowel. The reflexes of the labiovelars are generally indistinguishable from those of the plain velars in satem languages, but there are some words where the lost labialization has left a trace, such as by u-coloring the following vowel.

The centum group of languages, on the other hand, merged the palatovelars with the plain velar series , while the labiovelars were in general kept distinct. Centum languages show delabialisation of labiovelars when adjacent to *w (or its allophone *u), according to a rule known as the boukólos rule.

Fricatives The only certain PIE fricative phoneme was a strident sound. As is common for a lone sibilant, it was likely realized as a retracted sibilant. It had a voiced allophone that appeared before voiced obstruents, such as an ('nest'), and which later became phonemicized in some daughter languages. Some PIE roots have variants with appearing initially: such is called s-mobile.

The "laryngeals" may have been fricatives, but there is no consensus as to their phonetic realization. They are generally suspected to have been glottal, velar, uvular or even pharyngeal fricatives (much like the Semitic laryngeals that inspired their name), though at least some of these reconstructed phonemes have also been suggested to have been glottal or uvular stops originally.

Laryngeals

The phonemes (or and ), marked with cover symbol (also denoting "unknown laryngeal"), stand for three "laryngeal" phonemes.

The phonetic values of the laryngeal phonemes are disputable; various suggestions for their exact phonetic value have been made, ranging from cautious claims that all that can be said with certainty is that represented a fricative pronounced far back in the mouth and that exhibited lip-rounding, up to more definite proposals; e.g. Meier-Brügger writes that realizations of = , = and = or "are in all probability accurate". Another commonly cited speculation for is (e.g. Beekes). Simon (2013) has argued that the Hieroglyphic Luwian sign *19 stood for (distinct from ) and represented the reflex of . It is possible, however, that all three laryngeals ultimately fell together as a glottal stop in some languages. Evidence for this development in Balto-Slavic comes from the eventual development of post-vocalic laryngeals into a register distinction commonly described as "acute" (vs. "circumflex" register on long vocalics not originally closed by a laryngeal) and marked in some fashion on all long syllables, whether stressed or not; furthermore, in some circumstances original acute register is reflected by a "broken tone" (i.e. glottalized vowel) in modern Latvian.

The schwa indogermanicum symbol is sometimes used for a laryngeal between consonants, in a "syllabic" position.

Sonorants In a phonological sense, sonorants in Proto-Indo-European were those segments that could appear both in the syllable nucleus (i.e. they could be syllabic) and out of it (i.e. they could be non-syllabic). PIE sonorants consist of liquids, nasals and glides: more specifically, (or ) are non-labial sonorants, grouped with the cover symbol R, while labial sonorants (or ), are marked with the cover symbol M. All of them had syllabic allophones, transcribed , which generally were used between consonants, word-initially before a consonant, or word-finally after a consonant. Even though and were certainly phonetic vowels, they behave phonologically as syllabic sonorants. was an allophone of before velar consonants.

Reflexes

Some of the changes undergone by the PIE consonants in daughter languages are the following: Proto-Celtic, Albanian, Proto-Balto-Slavic and Proto-Iranian merged the voiced aspirated series with the plain voiced series . (In Proto-Balto-Slavic this postdated Winter's law. Proto-Celtic retains the distinction between and – the former became while the latter became .) Proto-Germanic underwent Grimm's law and Verner's law, changing voiceless stops into voiceless or voiced fricatives, devoicing unaspirated voiced stops, and fricativizing and deaspirating voiced aspirates. Grassmann's law ( > , e.g. > ) and Bartholomae's law ( > , e.g. > ) describe the behaviour of aspirates in particular contexts in some early daughter languages.

Sanskrit, Greek, and Germanic, along with Latin to some extent, are the most important for reconstructing PIE consonants, as all of these languages keep the three series of stops (voiceless, voiced and voiced-aspirated) separate. In Germanic, Verner's law and changes to labiovelars (especially outside of Gothic) obscure some of the original distinctions; but on the other hand, Germanic is not subject to the dissimilations of Grassmann's law, which affects both Greek and Sanskrit. Latin also keeps the three series separate, but mostly obscures the distinctions among voiced-aspirated consonants in initial position (all except become ) and collapses many distinctions in medial position. Greek is of particular importance for reconstructing labiovelars, as other languages tend to delabialize them in many positions.

Anatolian and Greek are the most important languages for reconstructing the laryngeals. Anatolian directly preserves many laryngeals, while Greek preserves traces of laryngeals in positions (e.g. at the beginning of a word) where they disappear in many other languages, and reflects each laryngeal different from the others (the so-called triple reflex) in most contexts. Balto-Slavic languages are sometimes valuable in reconstructing laryngeals since they are relatively directly represented in the distinction between "acute" and "circumflex" vowels. Old Avestan faithfully preserves numerous relics (e.g. laryngeal hiatus, laryngeal aspiration, laryngeal lengthening) triggered by ablaut alternations in laryngeal-stem nouns, but the paucity of the Old Avestan corpus prevents it from being more useful. Vedic Sanskrit preserves the same relics rather less faithfully, but in greater quantity, making it sometimes useful.

Vowels + Short Long Front Back Front Back Close (*) (*) Mid * * * * Open *? *?