Indo-European Phonetics
Short vowels
Chart of phonemic correspondences

IE *a *e *i *o *u *@
Celtic a e i, e (6) o u, o (7) a
Italic a e i o u a
Venetic a e i o u a
Hellenic a e i o u a (2)
Albanian a ie   a   a
Illyrian a e   a   a 
Thracian a e i a u a
Armenian a e, i   o (3) u a
Phrygian a e, i   a   a
Germanic a e, i (1) i a (3)   a
Slavic o e i o (3) u o
Baltic a e. i a (3) u a
Indic a a i a u i (4)
Iranian a a i a (3) u i
Anatolian a e, a (8)   o   a
Tocharian a e   o   á

Transcription of symbols:
@ - Indo-European 'schwa1'
i, u - Slavic super-short vowels are marked underlined
á, é, ó - long [a], [e], [o]
e. - Baltic open [e]
 

Notes:

1. IE *e > Common Germanic *e > Gothice, Western & Scandinavian i
2. IE *@ > Hellenic a; can be e, o under influence of long é, ó
3. IE *@ is dropped in the second (but non-finite) syllable in Armenian, Germanic, Baltic, Slavic, Iranian
4. IE *@ often disappears in Indic
5. IE *@ disappears everywhere before a vowel (Greek gen-e-tór, but gen-os)
6. IE *i > Celtic i, e (Gaulish i, Insular e)
7. IE *u > Celtic o, u (Irish u before a syllable containing u, i; in other cases o; Welsh u, Cornish o)
8. IE *e > Common Anatolian *e > Hittite e, Luwian & Palaic a
8. Please note that these are only root vowels
 

Examples:

1. IE *a
*arg- 'white, to shine'
*ag'- 'to lead, to bring'

2. IE *e
*gwen- 'a woman, a wife'
*newo- 'new'

3. IE *i
*kwis 'what, who'

4. IE *o
*ost- 'a bone'
*okw- 'an eye'

5. IE *u
*dhugh@tér- 'a daughter'
*(s)uper 'above', *upo 'under'

6. IE *@
*p@tér- 'father'
*dhugh@tér- 'a daughter'
 

Linguistic comment

After almost two centuries of Indo-European studies and research it seems that in Early Proto-Indo-European there was only one vowel which is sometimes marked V. Therefore, there could be two basic conditions of any Indo-European root: with the vowel V or without any vowel (zero grade) when the stress did not fall on the root syllable.

The vowel V later obtained three varieties depending on its phonemic situation in the word; those variants which in Late Proto-Indo-European turned into separate vowels e, o, zero and generated the system of Indo-European 'Ablaut' mutations - three grades like in Greek patros (zero grade), patera (e-grade), eupatora (o-grade). These three vowels became the basis of the Late Proto-Indo-European vocalic system: of all IE roots, about 92% are formed with e. After the laryngeal theory was invented, the emerging of e and o has been often regarded to as the original combinations of the phoneme V with the appropriate laryngeals: VH1 > eH1, VH3 > oH3.

Two vowels *i and *u in Proto-Indo-European were in fact sonant phonemes, having both vocal and consonant variants: *i - *y and *u - *w (sometimes they are all called 'semivowels' which is not quite correct). *i and *u  became *y and *w after a vowel, creating diphthongs.

The classical Indo-European system of short vowels was completed when the vowel *a emerged in the language: the origin of it, according to the most widespread opinion, is the neighborhood of the laryngeal phoneme H2, one of the three laryngeals in Proto-Indo-European which disappearec in all branches except Anatolian. The combination VH2 in the language gave finally aH2, and H2V > H2a respectively. This is the case in the roots like *arg'- (Indic arjuna-, Hittite harki- from Early PIE *H2arg'-) or *pás- (Latin pasco, Hittite *pahš- 'to breed' from Early PIE *paH2s-).

Proto-Indo-European *a, according to Meillet, was used in the roots only under some certain conditions, among which the most important were:
1. In words used for addressing people (like Hittite attaš 'father', cognate to Gothic atta 'father', Slavic otits 'father')
2. In expressive terms (like Old Indic kakhati 'he laughs') - including words denoting natural sounds
3. At the beginning of the word (normally like Latin ager 'field', cognate to Gothic akrs 'field')
4. In several words with unclear origin, which could be borrowed from non-Indo-European languages (like Latin faba 'a bean', barba 'a beard')

'Schwa', a super-short vowel of unclear timbre, appeared, as we may suppose, only in roots which contained the laryngeal phonemes, in unstressed positions. Therefore, it was a zero grade vowel only.

To summarize, the most widely known opinion about the origins of the Proto-Indo-European vocalism looks as follows:

1. V (the original vowel) became *e, *o, zero - depending on the stress and neighbouring phonemes in the root
2. *i and *u were vocalic variants of the sonant phonemes
3. *a appeared only next to or prior to the laryngeal H2 phoneme
4. *@ (schwa) was a zero grade vowel in roots, containing laryngeals

The Late Proto-Indo-European system was already much alike that of ancient European languages: there were five short vowels and the schwa. The structure remained practically the same in Latin and Ancient Greek, with only one important change - the schwa disappeared everywhere, turning into a in European languages and into i in the Indo-Iranian branch.

Another difference was the pronunciation of the Indo-European *o which was obviously more open than in Latin, Greek or modern IE tongues. This resulted to its change into a in a lot of languages (Balkan languages, Germanic, Baltic).

The vocalic system appeared completely destroyed in Indo-Iranian, where two main IE vowels, *e and *o, coincided with *a. But the transition *e > a is not unique here: a few Anatolian languages, Luwian and Palaic, also suffered this feature.