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 table uses the Wikipedia's canonical notation for transcribing Proto-Indo-European; variant transcriptions often seen elsewhere are provided for individual segments in the following sections.
PIE *p, *b, *bʰ are grouped with the cover symbol P. The phonemic status of *b 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.
They are symbolically grouped with the cover symbol T. 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: The actual pronunciation of these sounds in PIE is not certain.
In some phonological conditions, depalatalization occurred, yielding what appears to be a centum reflex in a satem language.
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 *ḱ, *ǵ, *ǵʰ with the plain velar series *k, *g, *gʰ, while the labiovelars *kʷ, *gʷ, *gʷʰ were in general kept distinct.
The only certain PIE fricative phoneme *s was a strident sound, whose phonetic realization could range from [s] or [θ] to palatalized [ɕ] or [ʃ].
The term laryngeal as a phonetic description is largely obsolete, retained only because its usage has become standard in the field.
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 *h₂ represented a fricative pronounced far back in the mouth, and that *h₃ exhibited lip-rounding up to more definite proposals; e.g. Meier-Brügger writes that realizations of *h₁ = [h], *h₂ = [χ] and *h₃ = [ɣ] or [ɣʷ] "are in all probability accurate".
[4] Another commonly cited speculation for *h₁ *h₂ *h₃ is [ʔ ʕ ʕʷ] (e.g. Beekes).
Simon (2013)[5] has argued that the Hieroglyphic Luwian sign *19 stood for /ʔa/ (distinct from /a/) and represented the reflex of *h₁.
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).
Some of the changes undergone by the PIE consonants in daughter languages are the following: 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.
Balto-Slavic languages are sometimes valuable in reconstructing laryngeals since they are relatively directly represented in the distinction between "acute" and "circumflex" vowels.
The surface vowels *i and *u were extremely common, and syllabic sonorants *r̥, *l̥, *m̥, *n̥ existed, but these sounds are usually analyzed as syllabic allophones of the sonorant consonants *y, *w, *r, *l, *m, *n.[6] The syllabic and non-syllabic versions of these sounds alternate in the inflectional paradigms of words such as *dóru ('tree, wood') (reconstructed with genitive singular *dréws and dative plural *drúmos) or in the derivation of words such as the noun *yugóm ('yoke') with *u, from the same root as the verb *yewg- ('to yoke, harness, join') with *w. Some authors (e.g. Ringe (2006)) have argued that there is substantial evidence for reconstructing a non-alternating phoneme *i in addition to an alternating phoneme *y as well as weaker evidence for a non-alternating phoneme *u.
[8] Any sonorant consonant can comprise the second part of a complex syllable nucleus; all can form diphthongs with any of the vowels *e, *o, *ē, *ō (such as *ey, *oy, *ēy, *ōy, *ew, *ow, *em, *en, etc.).
It is possible that Proto-Indo-European had a few morphologically isolated words with the vowel *a: *dap- 'sacrifice' (Latin daps, Ancient Greek dapánē, Old Irish dúas) or appearing as a first part of a diphthong *ay: *laywos 'left' (Latin laevus, Ancient Greek laiós, OCS lěvъ).
The following arguments can be set forth against recognizing *a as a phoneme of PIE: it does not participate in ablaut alternations (it does not alternate with other vowels, as the "real" PIE vowels *e, *o, *ē, *ō do), it makes no appearance in suffixes and endings, it appears in a very confined set of positions (usually after initial *k, which could be the result of that phoneme being a-coloring, particularly likely if it was uvular /q/), and words reconstructed with *a usually have reflexes in only a few Indo-European languages.
Albanian and Armenian are the least useful, as they are attested relatively late, have borrowed heavily from other Indo-European languages and have complex and poorly understood vowel changes.
A separate reflex of the original *o or *a is, however, argued to have been retained in some environments as a lengthened vowel because of Winter's law.
On the other hand, athematic nouns and verbs usually had mobile accent, with varied between strong forms, with root accent and full grade in the root (e.g. the singular active of verbs, and the nominative and accusative of nouns), and weak forms, with ending accent and zero grade in the root (e.g. the plural active and all forms of the middle of verbs, and the oblique cases of nouns).
Some nouns and verbs, on the other hand, had a different pattern, with ablaut variation between lengthened and full grade and mostly fixed accent on the root; these are termed Narten stems.
For example, for the noun *dʰéǵʰ-ōm, genitive *dʰǵʰ-m-és, Hittite has tēkan, tagnās, dagān and Tocharian A tkaṃ, tkan-, but these forms appear in Sanskrit kṣā́ḥ and Ancient Greek as khthṓn.
As was the case with the laryngeal theory, these cognate sets were first noted prior to the connection of Anatolian and Tocharian to PIE, and early reconstructions posited a new series of consonants to explain these correspondences.
Brugmann 1897's systematic explanation augmented the PIE consonant system with a series of interdentals (nowhere directly attested) appearing only in clusters with dorsals, *kþ *kʰþʰ *gð *gʰðʰ.
The conventional notations *þ *ð *ðʰ for the second elements of these metathesised clusters are still found, and some, including Fortson,[12] continue to hold to the view that interdental fricatives were involved at some stage of PIE.
An alternative interpretation (e.g. Vennemann 1989, Schindler 1991 (informally and unpublished)[13]) identifies these segments as alveolar affricates [t͡s d͡z].
Melchert has interpreted the Cuneiform Luwian īnzagan- 'inhumation', probably [ind͡zɡan], from *h₁en dʰǵʰōm 'in the earth', as preserving the intermediate stage of this process.
In any case, it is difficult to determine when a particular laryngeal loss is due to a protolanguage rule versus an instance of later analogy.