Indo-European Phonetics
Long vowels
Chart of phonemic correspondences


IE *á *é *í *ó *ú
Celtic á é, í (1) í á, ú (2) ú
Italic á é   ó  
Venetic á é í ó ú
Hellenic á, é (3) é í ó ú
Albanian á ó   é  
Illyrian       a  
Thracian á é, a í ú, ó ú
Armenian a i   u  
Phrygian a i   u  
Germanic ó é (4) í ó ú
Slavic a e. i a y
Baltic ó é í uo ú
Indic á á í á ú
Iranian á á í á ú 
Anatolian a e, i (2)   a  
Tocharic á e      

Transcription of symbols:
y - in Slavic a short closed vowel, reminds unlabialized [u]
á, é, í, ó, ú - long [a], [e], [i], [o], [u]
e. - Slavic open [e]
 

Notes:

1. IE *é > Celtic í and IE *ó > Celtic á in a stressed syllable
    IE *é > Celtic é and IE *ó > Celtic ú in unstressed syllables
2. IE *é > Common Anatolian *e, (nasal [e]) or *e
    Common Anatolian *e, > Hittite and Luwian i
3. IE *á > Common Hellenic *á > Aeolian & Doric *á, Attic & Ionian *é
4. Common Germanic is supposed to have had two *é:
    *é1 > Gothic e, Western & Scandinavian á (Gothic slepan, Old High German sláfan 'to sleep');
    *é2 > é everywhere (Old English hér, Gothic her 'here')
5. Please note that these are only root vowels
 

Examples:

1. IE *á
*bhrátér- 'a brother'
*stá- 'to stand'

2. IE *é
*dhé- 'to put, to set'
*p@tér- 'a father'

3. IE *í
*wíro- 'a man, a husband'
*lín- 'flax, linen'

4. IE *ó
*dó- 'to give'
*pó- / *pí- 'to drink'

5. IE *ú
*bhú- 'to be, to exist'
*sú- 'to give birth'
 

Linguistic comment

There was obviously five long vowels in Proto-Indo-European. But originally the opposition between long and short vocals did not exist: long vowels begin to appear somewhere in the middle of the Proto-Indo-European epoch.

The source for them was, as we stated in the comment to the short vowels section, the laryngeal phonemes which disappeared in the majority of Indo-European dialects lengthening the preceding or the following vowels: Latin dó 'I give', Hittite dahhi 'I take' < *doH3-; Latin ára 'furnace', Hittite hašša- 'furnace' < *H2as-.

Therefore in Late Proto-Indo-European the system of short vowels generated the respective long sounds. This opposition was important in the Indo-European 'ablaut' mutation, where it emerged under the influence of several phonetic features. For example, long vowels are common in the last root syllable in the nouns of consonant stems (like *p@tér- 'father', *ped- 'a foot', *reg'- 'to direct'): before the ending *-s in nominative singular their vowel always became long:

Latin réx 'king' < *reg'-s, but genitive singular regis 'of a king';
Greek patér 'father' < *p@ter-s, but accusative singular patera;
Sanskrit pát 'foot' < *ped-s, but genitive singular padas 'of a foot'.

This compensatory lengthening is typical for a number of Indo-European tongues, so it is easy to suppose that it was also frequent in the Proto-language. A similar situation existed in the verb system: the Indo-European aorist marker *-s- often caused lengthening of the preceding root vowel of the verb:

Latin vehó "I carry' < *wegh-ó; 1st person singular perfect véxí 'I carried' < *wegh-s-.
Common Slavic *reko. 'I speak' < *rek-, but aorist *re.ch- 'said' < *rék-s-

By the time the Indo-European language community breaks up, long and short vowels were independent phonemes which appeared in different fixed positions and denoted different grades of the root 'ablaut' mutation.

Summary:
1. Long vowels emerged first as phonological variants of their short counterparts.
2. Among the conditions which caused the formation of long vowels the most important were neighbouring laryngeal phonemes, and compensatory lengthening before certain elements of the nominal and verbal structures.

Practically in all Indo-European branches long vowels were preserved. The exception is evidently Hittite where no oppoition between long and short vocals can be traced. Among the modern Indo-European languages Slavic, Romance, Armenian, Greek and many Indo-Iranian languages lost this opposition as well.