The traditional reconstruction of Proto-Indo-European includes the following stop consonants:
*b is parenthesized because it is at best very rare and perhaps nonexistent.
Historically, the inventory was not introduced as an independent proposal but instead arose as a modification of an earlier, typologically more plausible theory.
In the original Proto-Indo-European proposal, there was a fourth phonation series, voiceless aspirated *pʰ, *tʰ, *ḱʰ, *kʰ, *kʷʰ, assumed to exist on the basis of what is found in Sanskrit, which was then thought to be the most conservative Indo-European language. However, it was later realized that this series was unnecessary, and it was generally the result of a sequence of a tenuis stop (*p, *t, *k, *ḱ, *kʷ) and one of the Proto-Indo-European laryngeal consonants, in other words either *h₁, *h₂, or *h₃. The aspirate series was removed, but the breathy voiced consonants remained.
There are several problems with the traditional reconstruction. The first is the rarity of *b. From a typological point of view, if a single voiced stop is missing from a phoneme inventory (a 'gap'), it would normally be /ɡ/ that is missing; on the other hand, if a voiceless stop is missing, the labial /p/ is the most likely candidate.
Secondly, there are few languages which have breathy voiced consonants but no voiceless aspirates and even fewer that simultaneously contrast breathy voice with full voice. Roman Jakobson has asserted that no such language is known; however, that is disputed by some linguists who oppose the theory. For example, Robert Blust showed that a system of voiceless, voiced and voiced aspirated (not murmured) stops, as postulated in the traditional reconstruction of Proto-Indo-European, exists in Kelabit, a language of the Sarawak highlands in Borneo. Others have observed, however, that the actual phonetics involved in Kelabit is not murmured but voiceless with breathy release and so is not comparable to what is posited for Proto-Indo-European. In any event, the traditional reconstruction remains a typological oddity.
The third issue is a longstanding but unexplained observation of Indo-Europeanists about the distribution of stops in word roots. It had long been noted that certain combinations of consonants were not represented in Proto-Indo-European words in terms of the traditional system:
- No root contained a sequence of two plain voiced stops: there were no roots of the type **deg.
- No root contained both a voiceless stop and a voiced aspirate: roots of the type **dʰek or **tegʰ were not attested.
- On the other hand, the plain voiced stops were compatible with either of the other two series: *degʰ or *dek were both possible.
The constraints on the phonological structure of the root cannot be explained in terms of a theory of assimilation or dissimilation since they display a radical difference in patterning between two sets of consonants (the voiced stops) that ought to behave identically. Typologically, it is also very odd.
The glottalic theory proposes different phonetic values for the stop inventory of Proto-Indo-European:
In his version of the glottalic theory, Hopper (1973) also proposed that the aspiration that had been assumed for the voiced stops *bʰ *dʰ *gʰ could be accounted for by a low-level phonetic feature known to phoneticians as "breathy voice". The proposal made it possible both to establish a system in which there was only one voiced stop and, at the same time, to explain developments in later Indo-European dialects (which became Greek, Latin, and Sanskrit) that pointed to some kind of aspiration in the voiced series. Hopper also treated the traditional palatalized vs. plain velar dichotomy as a velar-uvular contrast.
Gamkrelidze and Ivanov (1973, 1995:5-70) have posited that both non-ejective series (traditional *p *t *k and *bʰ *dʰ *gʰ) were fundamentally aspirated (that is, *pʰ *tʰ *kʰ and *bʰ *dʰ *gʰ, respectively) but had non-aspirated allophones (that is, *[p] *[t] *[k] and *[b] *[d] *[g]). According to them, the non-aspirated forms occurred in roots where two non-ejectives were present because of a rule that prohibited more than one aspirate in the same root. To express the variability of aspiration, Gamkrelidze and Ivanov write it with a superscripted h, for example *dʰ. Thus, an Indo-European *DʰeDʰ (where *Dʰ represents any non-ejective stop) might be realized as *DeDʰ (attested in Indic and Greek) or as *DʰeD (attested in Latin). In contrast, traditional theory would trace a form attested as both *DeDʰ and *DʰeD to an Indo-European *DʰeDʰ. The advantage of this interpretation over the previous is circumventing the typological oddity of the language having only voiced aspirates by identifying the voiceless non-aspirates of the traditional stop system (*p *t *k) as voiceless aspirates (*pʰ *tʰ *kʰ).
The phonation system proposed by the glottalic theory is common among the world's languages. Moreover, the revised system explains a number of phonological peculiarities in the reconstructed system. The absence of a labial plain voiced stop *b in the protolanguage now becomes an absence of a labial ejective *pʼ, proportionally a rather more common state of affairs. The theory also provides a completely coherent explanation to the patterning of the stop series in roots (Hopper 1973):
- In many languages that have glottalized consonants, there is a phonetic constraint against two such consonants in the same root. The constraint has been found in many languages of Africa, the Americas, and the Caucasus. In Akkadian, the constraint affected borrowed and inherited roots, and one of the two heterorganic emphatics undergoes dissimilation and appears as a simple (unmarked) consonant, which is known as Geers' law.
- If the "plain voiced stops" were not voiced, the "voiced aspirated stops" were the only voiced stops. The second constraint can accordingly be reformulated as two nonglottalic stops must agree in voicing.
- Since the glottalic stops were outside the voiced/voiceless opposition, they were immune from the constraint on voicing agreement in (2).
In 1981, Hopper proposed to divide all Indo-European languages into Decem and Taihun groups, according to the pronunciation of the numeral '10', by analogy with the Centum-Satem isogloss, which is based on the pronunciation of the numeral '100'. The Armenian, Germanic, Anatolian, and Tocharian subfamilies belong to the Taihun group because the numeral '10' begins with a voiceless t there. All other Indo-European languages belong to the Decem group because the numeral 10 begins with a voiced d in them. The question then can be framed as which, if either, of these groups reflects the original state of things and which is an innovation.
While the glottalic theory was originally motivated by typological argument, several proponents, in particular Frederik Kortlandt, have argued for traces of glottalization being found in a number of attested Indo-European languages or the assumption of glottalization explaining previously known phenomena, which lends the theory empirical support. (Similarly, the laryngeal theory was proposed before direct evidence in Anatolian was discovered.)
Among the Indo-Iranian languages, Sindhi reflects the non-aspirated voiced series unconditionally as implosives. Kortlandt also points out the distribution of voiced aspirates within Indo-Iranian: they are lacking from the Iranian languages and the Nuristani languages, two of the three accepted main branches of Indo-Aryan, and within the third, Indo-Aryan, also lacking from Kashmiri, which he suggests points to voiced aspirates being an innovation rather than a retention.
In Germanic, Danish stød in certain dialects (vestjysk stød) corresponds with the Proto-Germanic voiceless stops, deriving from the allegedly glottalized PIE series. Kortlandt also proposes word-final glottalization in English to be a retention and derives features such as preaspiration in the Scandinavian languages and certain instances of gemination in High German from preglottalization as well.
In both Latin (Lachmann's law) and Balto-Slavic (Winter's law), vowels are lengthened before a "voiced" consonant. It is the same behavior that vowels exhibit before Proto-Indo-European laryngeals, which are assumed to have included a glottal stop. It may be that the glottalic consonants were preglottalized or that they were ejectives that became preglottalized in Italic and Balto-Slavic before losing their glottalization and becoming voiced. It is very common in the world's languages for glottal stops to drop and lengthen preceding vowels. In Quileute, for example, the sequences VCʼV, VʔCʼV, and VːCʼV, as found for example in ak’a ~ a’k’a ~ āk’a, are allophones in free variation.
In Balto-Slavic, glottalization is also directly attested, in the broken tone of Latvian and Žemaitian.
Dialects of Armenian also show glottalization. It has been argued to be influence from the other Caucasian languages, but Kortlandt argues glottalization cannot be considered a modern innovation and must be reconstructed with a wider dialectal distribution in older stages of Armenian.
The primary objection to the glottalic theory is the alleged difficulty in explaining how the sound systems of the attested dialects were derived from a parent language in the above form. If the parent language had a typologically unusual system like the traditional p b bʰ, it might be expected to collapse into more typical systems, possibly with different solutions in the various daughter languages, which is what one finds. For example, Indo-Aryan added an unvoiced aspirate series (/pʰ/), gaining an element of symmetry; Greek and Italic devoiced the murmured series to a more common aspirate series (*bʰ to /pʰ/); Iranian, Celtic, and Balto-Slavic deaspirated the murmured series to modal voice (*bʰ to /b/); and Germanic and Armenian chain-shifted all three series (*p *b *bʰ > /f p b/). In each case, the attested system represents a change that could be expected from the proposed parent. Now, if the system were typologically common, as proposed by the glottalic theory, it might be expected to be stable and therefore preserved in at least some of the daughter languages, which is not the case: no daughter language preserves ejective sounds where the glottalic theory postulates them. Glottalic proponents respond that if Proto-Indo-European did not have true ejectives but some less stable kind of glottalic consonant, their loss would be more understandable; but that undercuts many of the original motivations of the glottalic theory, which are based on ejectives (rather than glottalized consonants) and on the idea of a typologically natural (hence stable) system. Regardless, there are languages in which ejective consonants have voiced allophones, such as Blin and Kw'adza, which has been suggested as an "empirical precedent" for the glottalic theory.
The typological underpinnings of the glottalic theory itself have also been questioned, for instance in 2002-3 by Barrack.
Additionally, if traces of glottalic stops can be found in separate branches such as Italic and Indo-Iranian, the change of *p’ *t’ *k’ to *b *d *g must have occurred independently in each branch after their separation from the Proto-Indo-European matrix. Taking them as identical but independent innovations would, according to traditional models of sound change, be an astonishing coincidence, which most linguists would find very hard to believe because that ejectives tend to be quite stable diachronically. However, it cannot be assumed that Proto-Indo-European was a uniform language and presumably, a putative shift from ejective to voiced stops was already present as variation at an early stage. Kortlandt also asserts that the change from aspirated to plain voiced stops, which is likewise required as an independent change in numerous Indo-European branches under the traditional model, is not attested elsewhere and is typologically suspect. (However, the exact change has been observed to have taken place independently numerous times in the Indic languages.)
A compromise viewpoint would be to see the original formulation of glottalic theory (with ejective stops) as representing an earlier stage in the history of Proto-Indo-European, which would have undergone a period of internal evolution into a stage featuring unstable voiced glottalized stops before branching out into the daughter languages. That would explain the root restrictions in Proto-Indo-European, the near-universal loss of glottalic consonants in the daughter languages and the lack of *b in the traditional system.
A scenario of glottalic framework in pre-Proto-Indo-European, although possible, is at present unprovable by the methods of historical linguistics because of the uncertainty concerning the possibility of other languages or language families being related to Proto-Indo-European, which might be used as corroborating evidence; and in practical terms, it is irrelevant for the traditional reconstruction of Proto-Indo-European that describes only its latest stage (the so-called "Late Proto-Indo-European"). However, Kortlandt suggests that voiced aspirate was probably not in Indo-European before the division into the branches.
Fallon has reviewed and discussed the arguments for and against the ejective model of Proto-Indo-European consonantism and has concluded that most of the objections raised against the glottalic theory are specious.
The oldest stratum of Iranian loanwords to Armenian demand consonant shifts from voiced to voiceless, which are not possible in a glottalic theory framework in which they were voiceless to begin with. Compare:Iran. *ardzata- > Old Armenian arcatʿ "silver"
The same argument is valid for early Proto-Celtic borrowings into Proto-Germanic, such as Celt. *rīg- > Germ. *rīks
Additional evidence from Armenian comes in the form of Adjarian's law, according to which in certain dialects initial-syllable vowels are fronted after consonants reflecting the inherited (PIE) voiced aspirates. The conditioning is not a synchronic process but reflects the quality of the original prevocalic consonant. It further demonstrates that Proto-Armenian retained Proto-Indo-European stops. Since voiced aspirates would then have to be reconstructed for Proto-Armenian, only Germanic could be claimed to be archaic with respect to the traditional voiced aspirate series in the traditional glottalic theory framework.
One objection that has been raised against the glottalic reconstruction is that the voiced stops are voiceless in some daughter languages: "unvoiced" in Tocharian and Anatolian, and aspirates (later, fricatives) in Greek and Italic. Thus, some more recent versions of the Glottalic Theory hypothesis do not have voiced consonants at all, or treat voicing as non-distinctive. For example, Beekes describes the traditional voiced series as pre-glottalized instead of ejective. This is based on the "voiced" series triggering length in preceding vowels in daughter languages, the glottalic closure before the stop acting in a manner akin to the laryngeals. This analysis results in the following phoneme inventory:
Another alternative to the glottalic theory proposed by James Clackson bases the contrast on phonation. Observing that the traditional voiced aspirated series of PIE is preserved in languages like Sanskrit not as true voiced aspirates but as voiced consonants with breathy or murmured voice, Clackson suggests the contrast between voiceless, voiced, and voiced aspirates could be reframed as stops conditioned by three phonations: voiceless, creaky or stiff voice, and breathy voice. This, he argues, is typologically more common than voiced aspirates without voiceless counterparts. Along these lines, Schirru has also suggested that the voiced aspirated stops could be better analyzed as having the feature [+slack vocal folds] or [-stiff vocal folds]. Note that interpretations of reconstructed PIE voiced stops as preglottalized and as creaky stops are not mutually exclusive, since glottal closure is often realized as creaky phonation on neighboring sounds.