The Greek language underwent pronunciation changes during the Koine Greek period, from about 300 BC to 400 AD. At the beginning of the period, the pronunciation was close to Classical Greek, while at the end it was almost identical to Modern Greek.
Vowel length distinctions are important for classical poetry and drama, but become less important for prose into the patristic age.
Overview
The most significant changes during the Koine Greek period concerned vowels: these were the loss of vowel length distinction, the shift of the Ancient Greek system of pitch accent to a stress accent system, and the monophthongization of diphthongs (except and ). These changes seem widely attested from the 2nd century BC in Egyptian Greek, and in the early 2nd century AD in learned Attic inscriptions; it is therefore likely that they were already common in the 2nd century BC and generalized no later than the 2nd century AD.
Another change was the frication of the second element of diphthongs and . This change likely took place after the vocalic changes described above occurred. It is attested in Egyptian Greek starting from the 1st century AD, and seems to have been generalized in the late Roman period.
Another series of changes was the frication of voiced stops, which is widely attested in Egyptian Greek starting from the 1st century AD, but may have been generalized at a later date, possibly in the late Roman or early Byzantine periods.
Yet another series of changes was the frication of aspirated voiceless stops, which is attested in several locations from the 1st century AD, but seems to have been generalized at a later date, possibly in the late Roman or early Byzantine period.
A last change (possibly related to frication of aspirated stops) is the loss of , which may have begun as soon as the late 1st century BC in Egyptian Greek, seems to have taken place no earlier than the 2nd century AD in learned Attic inscription, and had most probably been generalized by the late Roman times.
Controversies about reconstructions
The primary point of contention comes from the diversity of the Greek-speaking world: evidence suggests that phonological changes occurred at different times according to location and/or speaker background. It appears that many phonetic changes associated with the Koine period had already occurred in some varieties of Greek during the Classical period.
An opposition between learned language and vulgar language has been claimed for the corpus of Attic inscriptions. Some phonetic changes are attested in vulgar inscriptions since the end of the Classical period; still they are not generalized until the start of the 2nd century AD in learned inscriptions. While orthographic conservatism in learned inscriptions may account for this, contemporary transcriptions from Greek into Latin might support the idea that this is not just orthographic conservatism, but that learned speakers of Greek retained a conservative phonological system into the Roman period. On the other hand, Latin transcriptions, too, may be exhibiting orthographic conservatism.
Interpretation is more complex when different dating is found for similar phonetic changes in Egyptian papyri and learned Attic inscriptions. A first explanation would be dialectal differences (influence of foreign phonological systems through non-native speakers); changes would then have happened in Egyptian Greek before they were generalized in Attic. A second explanation would be that learned Attic inscriptions reflect a more learned variety of Greek than Egyptian papyri; learned speech would then have resisted changes that had been generalized in vulgar speech. A last explanation would be that the orthography in learned Attic inscriptions was artificially conservative; changes may then have been generalized no later than they are attested in Egyptian papyri. All these explanations are plausible to some degree, but would lead to different dating for the generalization of the same changes.