Ancient Greek phonology

Differences from Proto-Indo-European
Ancient Greek differs from Proto-Indo-European and other Indo-European languages in certain ways. In phonotactics, Ancient Greek words could only end in a vowel or ; final stops were lost, as in γάλα "milk", compared with γάλακτος "of milk" (genitive). Ancient Greek of the classical period also differed in phonemic inventory:
 * PIE became  at the beginning of a word (debuccalization): Latin sex, English six, Ancient Greek ἕξ.
 * PIE was lost between vowels via an intermediate step of debuccalization: Sanskrit , Latin generis (where s > r by rhotacism), Greek *genesos > *genehos > Ancient Greek γένεος, Attic γένους  "of a kind".
 * PIE  became  (debuccalization) or  (fortition): Sanskrit , Ancient Greek ὅς "who" (relative pronoun); Latin iugum, English yoke, Ancient Greek ζυγός.
 * PIE, which occurred in Mycenaean and some non-Attic dialects, was lost: early Doric ϝέργον, English work, Attic Greek ἔργον.
 * PIE and Mycenaean labiovelars changed to plain stops (labials, dentals, and velars) in the later Greek dialects: for instance, PIE became  or  in Attic: Attic Greek ποῦ  "where?", Latin quō; Attic Greek τίς, Latin quis "who?".
 * PIE "voiced aspirated" stops were devoiced and became the aspirated stops φ θ χ  in Ancient Greek.

Phonemic inventory
The pronunciation of Ancient Greek was very different from that of Modern Greek. Ancient Greek had long and short vowels; many diphthongs; double and single consonants; voiced, voiceless, and aspirated stops; and a pitch accent. In Modern Greek, all vowels and consonants are short—many vowels and diphthongs once pronounced distinctly are pronounced as (iotacism). Some of the stops and glides in diphthongs have become fricatives, and the pitch accent has changed to a stress accent. Many of these changes took place in the Koine Greek period. The writing system of Modern Greek, however, does not reflect all pronunciation changes.

The examples below represent Attic Greek in the 5th century BC. Ancient pronunciation can never be reconstructed with certainty, but Greek from this period is well documented and there is little disagreement among linguists as to the general nature of the sounds that the letters represent.

Consonants
occurred as an allophone of used before velars and as an allophone of  before nasals. was probably voiceless when word-initial (written ῥ). was assimilated to before voiced consonants.

Vowels
raised to, probably by the 4th century BC.