The Phonology of Proto-Indo-European

C. George Boeree

Although it is impossible to know how PIE was actually pronounced before it developed into its daughter languages, linguists have attempted to reconstruct more likely possibilities. The assumption is that there is some consistency to the changes that took place (phonological "laws") in the daughter languages, and that the pronunciation can be determined by working backwards. This article explains some of the results and adds a few comments as well.


The traditional set of consonants included the following plosives:


gʲʰ gʷʰ
The plosives with ʲ are palatalized, i.e. pronounced with a slight j (English y) sound; those with ʷ are labialized, i.e. pronounced with a slight w sound. Those with ʰ are aspirated, meaning the b, d, g and gʷ are followed by a slight "breathy" h. This distinction is important in some languages (such as many in India), but is absent in English.

Because the palatal group of velar plosives (kʲ, gʲ, and gʲʰ) is very common, and the plain group (k, g, and gʰ) is very rare and may be accounted for in other ways, many now prefer a simpler set, with the plain set entirely replacing the palatals:
Notice that the b is in parentheses. That is because b is extremely rare in PIE, and may, in fact, not even truly exist. This is a very unusual situation. If any labial plosive is missing in a language, it will be the p, not the b. For this reason, the Russian linguist Diakonov suggested that the voiced set of plosives were really ejectives, that is, the sound is followed by a strong release of air from the glottis, indicated by a following apostrophe. This has led to the glottalic theory, which lists the plosives so:
t’ k’ k’ʷ
Notice that the first row of plosives have become aspirated or "breathy". We can see this in the difference between the sounds of p, t, k, and qu in spin, steep, skip, and squeeze and those in pool, tool, cool, and quo, which are actually aspirated, even though we never think about it.

I will be bold and suggest another possibility:
kʲʰ kʷʰ
gʲʰ gʷʰ
In other words, the traditional first line is unvoiced and pronounced with aspiration; the second is pronounced unvoiced without aspiration, and the last line is pronounced with voice and aspiration. This is a common set of plosives as languages go, and easily permits the near absence of the p. Also, nearly all languages that have voiced aspirated plosives (e.g. dʰ) also have unvoiced aspirated plosives (e.g. tʰ).

In support of this, note that aspirated p, t, and k are retained in Armenian and become parallel fricatives in Germanic, while in most other daughter languages they are deaspirated. Plain p, t, and k of the middle line are retained in Armenian and Germanic, while they are devoiced in others. And aspirated b, d, and g are retained in Sanskrit, while devoiced in Greek and deaspirated in the rest. Tokharian apparently lost all aspiration and voiced-unvoiced distinctions among plosives. Comparatively, devoicing and deaspiration are more common that the reverse.

Among the velar plosives, I suggest that the distinctions between kʲʰ/kʲ/gʲʰ and kʷʰ/kʷ/gʷʰ are similar to Irish slender (lips as if to say ee) vs broad (lips as if to say oo).

In western PIE (eg. Celtic, Italic, and Germanic), the slender velars lost their slight y sound, and the broad became more clearly kw and gw. In eastern PIE (eg. Balto-Slavic and Indo-Iranian), the broad velars lost their slight w sound, and the slender velars became ɕ, ʃ, or s and ɟ, ʒ, or z. The western languages became known as centum languages (from the Latin word for 100, pronounced kentum); the eastern languages became known as the satem languages (from the Avestan word for 100). Tokharian, and Greek are also considered centum languages, and Hittite may have been. Armenian and Albanian are considered satem.

Grimm's Law (regarding changes in the pronounciation of plosives in Germanic) looks a little different if one uses this scheme, and no longer requires a "cascade" of changes (wherein the first changes, from aspirated unvoiced plosives to fricatives, are the most unusual, and the final changes, from aspirated voiced plosives to unaspirated voiced plosives, occurs in most of the Indo-European daughter languages):
pʰ > ɸ > f tʰ > θ kʲʰ  > x > h
kʷʰ > xʷ > hw
t (no change) kʲ > k
kʷ > kw
bʰ > b dʰ > d gʲʰ > g gʷʰ > gw > w
Outside of Germanic (and, to an extent, Armenian), the changes look something like this (including the centum / satem distinctions):
pʰ > p
tʰ > t
kʲʰ > k / ɕ (etc.)
kʷʰ > kw / k
t > d
kʲ > g /  ɟ (etc.) kʷ > gw / g
bʰ > b
dʰ > d
gʲʰ > g /  ɟ (etc.) gʷʰ > gw / g
Sanskrit, of course, retains the original aspirations for the first and last row. Next, here are examples of PIE words (with my versions in parentheses) and their English descendents:
*pōds, *ped (pʰōds, pʰet)
      - foot
*tréyes (tʰréyes)
      - three
*ḱwon (kʲʰwon)
      - hound
*kʷid, kʷod (kʷʰid, kʷʰod)
      - what
 - *déḱm̥t (tékʲm̥t)
      - ten
*ǵénu, ǵnéus (kʲénu, kʲnéus)
      - knee
*gʷih₃wo (kʷih₃wo) (alive)
      - quick
*bʰréh₂ter (bhreh₂ter)
      - brother
*dʰugh₂-tér (dʰugh₂ter)
      - daughter
*ǵʰans (gʲʰans)
      - goose
*gʷʰerm (gʷʰerm)
      - warm
(Germanic words with a "p" were borrowed from non-Germanic languages, for the most part.)

 Before moving on to the vowels, I must mention the rest of the consonants believed to have been a part of PIE: s, m, n, l, r, j (sometimes written i̯ or y), and w (sometimes written u̯). These have remained remarkably stable in most of the daughter languages.


Just as the consonants have different possible systems, so do the vowels. The traditional system postulates five vowels, which can be short or long (e, o, a, i, u, ē, ō, ā, ī, ū). In addition, there are 12 diphthongs (ei, oi, ai, eu, ou, au, ēi, ōi, āi, ēu, ōu, āu). Also included in many interpretations is the central vowel ə, possibly pronounced like the a in ago.

When it became clear that Hittite (as well as the other Anatolian languages) was in fact a relative of the other Indo-European languages, a new way of looking at the vowels arose: The basic vowels in this system are e, ē, o, ō (with a possible a and ā), plus vowel versions of w and j (i.e. u and i). The evidence from Hittite suggests that a set of three laryngeals (consonants pronounced somewhere in the back of the mouth), labelled h₁, h₂, and h₃, affected the pronunciation of these vowels. Notably, the laryngeals altered the pronunciation of e, changing it to a when next to h₂ and to o when next to h₃, and lengthening the vowels when they follow them. When a laryngeal occured between two consonants, it would have been pronounced as ə.

Suggestions as to what these laryngeals may have sounded like include the following:
h₁: [ʔ] [h] [ç]
h₂: [ħ] [ʕ] [ʜ] [ʢ] [χ] [x]
h₃: [ʕʷ] [ʢʷ] [ɣʷ][ xʷ]
In my (amateur) opinion, the laryngeals may have been pronounced as hʲ, h, and hʷ. I make these suggestions because Hittite was most likely to retain an h where h₂ (and sometimes h₃) had been and, in a few daughter languages, you can find i where h₁ had been and u where h₃ had been. I would expect that the hʲ sound would move the vowel towards i and the hʷ  towards u, thereby altering ə to e and o respectively. The progression, then, from pre-PIE (PIE before Hittite split off from the rest) to Balkan PIE might look like this:
h₁e > hʲə > e
h₂e > hə > a
h₃e > hʷə > o

eh₁ > əhʲ > ē
eh₂ > əh > ā
eh₃ > əhʷ > ō
(I prefer to use ə with laryngeals, to allow movement to the front for e, to the back for o, and down for a.)

Only Hittite preserves some indication of these laryngeals, and only Greek consistently shows all three vowels resulting from their effects. This leaves me feeling quite skeptical of laryngeal theory, but most of the professionals insist that it is very useful in understanding the evolution of the daughter languages, so who am I to disagree. It is, I must admit, not difficult to explain the vowels in all IE daughter languages by beginning with only one vowel: ə!

Both systems include the possibility of using l, m, n and r as syllabic consonants which function like vowels. These are written l̥ n̥, m̥, and r̥, and are pronounced a little like they often are in unstressed endings in English such as those in little, mitten, and bitter. In the laryngeal approach, these too can be affected by the laryngeals:
iH > i: (H represents any of the laryngeals)
uH > ū
l̥ = lə
l̥l = əl
l̥h₁ = ləhʲ > lē
l̥h₂ = ləh > lā
h₃ = ləhʷ > lō
The same pattern applies to n̥, m̥, r̥. Note, however, that these distinctions are only truly found in Greek. In all other branches of PIE,  l̥H > l̥: > Vl/lV, that is, the presence of a laryngeal after a syllabic consonant only lengthens the syllable, and the daughter languages add a vowel (V) before or after the l, n, m, or r.

Once again, we will never know how these prehistoric people spoke. But it does make for an interesting linguistic puzzle!

© 2014, C. George Boeree