Greek Koine had at least two variants. Literature forms of it were based mainly on the Attic classical tongue, they are known by works by Polibius, Epictetus and others. The colloquial Koine was expressed in thre language of the Greek Bible.
Greek Koine had some characteristics which reflect another large step in the development of Greek in whole. The vowels gradually lose their opposition of long and short ones. Diphthongs are also simplified greatly, and therefore the system of vowels becomes very simple, unlike the Ancient Greek one. The aspiration (< Indo-European s-) at the beginning of the word disappears. Consonats are becoming simpler as well: dz > z, aspirated consonants ph > f, kh > ch, th > þ (English unvoiced [th]). A new sound ts appeares.
The morphology also acquires some changes in this period. The so-called second Attic declension falls out of use, so leós > laos, and the 2nd declension loses all its feminine nouns as parthenos (a girl) > parthena. The vocative case coincides with nominative (as in all late Indo-European languages), and the dual number - with plural, also a common process. Moreover, the dative case gradually disappeares. In general, many synthetic traits of such an ancient Indo-European as was Greek, are replaced with analytical forms in Koine. For instance, the suppletive comparative forms and all synthetic comparatives disappear here.
Koine is the grade lying between the Ancient Greek and the New Greek language.