Obstruents are stop consonants. Both lips may stop the flow of air from the lungs (in bilabial articulation), or the tongue tip may stop the air flow at the teeth (in dental articulation), or the mid to back tongue area may stop air flow against the roof of the mouth. The term palatal refers to stopping the air at the "palate" (the hard upper part of the mouth), as in English 'kitten', and the term velar refers to stopping air at the velum (the softer part farther back), as in English 'cough'.
It is debated whether the palatal vs. velar stop made a difference in meaning in Proto-Indo-European (PIE), or whether the following vowel was responsible for meaning differences, as it is in English. With words where palatal vs. velar obstruents are thought to be distinctive, palatals are indicated by a diacritic. Otherwise, the sound is assumed to be velar or palato-velar.
Palato-velar stops might also be labialized, resulting in stop consonants called labio-velars: [kw] as in English 'quick' and [gw] as in 'Guantanamo'. Another feature of stop co-articulation besides labialization is glottalization, a simultaneous flexing of the glottis to make a sound that "ejects" the air as well as stopping it.
In addition to the place where the air may be stopped, and features of co-articulation such as labialization, obstruents might also be voiced or unvoiced (voiceless). For voiced stops, the vocal cords vibrate at the same time that the air is stopped; for voiceless stops, they do not.
|Voiceless (aspiration optional)||P = P(h)||T = T(h)||K = K(h)||Kw = Kw(h)|
|Voiced ~ Voiceless glottalized||B = P'||D = T'||G = K'||Gw = K'w|
|Voiced (Aspirated)||Bh = B(h)||Dh = D(h)||Gh = G(h)||Gwh = Gw(h)|
* In the table above, palatal and velar obstruents have been collapsed into a single column; distinctions between these types may be indicated elsewhere.
Currently, two major hypotheses propose reconstructed systems of PIE sounds. The "traditional" hypothesis, which goes back to 19th century work exemplified in Lehmann's Reader, reconstructs three sets of stops: voiceless [p t k kw], voiced [b d g gw], and voiced aspirated [bh dh gh gwh]. This system was deduced for PIE via the comparative method, and is represented by entries on the left side of '=' signs in the table above. The late 20th century Glottalic Theory, by contrast, also takes into consideration findings about the types of sound systems that actually occur in the world's languages (see the section on phonology in Thomas V. Gamkrelidze & Vjaceslav V. Ivanov, Indo-European and the Indo-Europeans [1984 Russian original translated by Johanna Nichols; Berlin: de Gruyter, 1995, 2 vol's]).
As it turns out, the traditional consonant system reconstructed for PIE in the 19th century (with [b] missing from the PIE language) is statistically atypical of the world's languages, while the glottalic system (with [p'] missing from PIE), as represented by entries on the right side of '=' signs in the table above, is much more common and has parallels in languages such as those of the Caucasus (e.g., Georgian).
As shown by '=' signs in the table above, traditional voiceless stops correspond to glottalic voiceless stops, which are reconstructed with optional or allophonic aspiration; this views PIE aspiration as similar to that in English, where the [ph] of 'pot' contrasts only subliminally with the unaspirated [p] of 'spot'. Similarly, traditional voiced stops correspond to voiceless glottalized stops [p', t', k', k'w], and traditional voiced aspirated stops correspond to glottalic voiced stops with optional aspiration.
Proto-Indo-European forms, as reconstructions, all belong to the realm of hypothesis. Although we no longer doubt that there must have been a proto-language spoken by a prehistoric Indo-European speaker group, the exact phonological system of that prehistoric language cannot be proved in the same way that mathematical theorems are said to be proved. Our examples may show both reconstructions in order to facilitate study of the implications of each, and to make reference works that use one or the other more accessible.