During the period of the Byzantine Empire (i.e., until the fall of Constantinople in 1453) the language of administration and of most writing was firmly rooted in the Atticist tradition; it is this archaizing style that is often referred to as “Byzantine Greek.” The spoken language continued to develop apace, however, and its course can be followed to some extent in the writings of the less-educated chroniclers (such as John Malalas, 6th century) and hagiographers. Furthermore, the increasing political disintegration characteristic of the last few centuries preceding the fall of Constantinople brought with it a general decline in educational level, and works appeared that reflect quite closely the colloquial language of the time, although learned and pseudo-learned elements are never absent. While the differences between the Chronicle of the Morea (13th century), for example, and present-day spoken Greek are quite minor, Byzantium failed to produce a writer of the stature of Dante, capable of establishing once and for all the living vernacular as a worthy vehicle for great literature.
Most of the phonological and grammatical developments that separate present-day Greek from the Koine occurred during this period. Thus, in the phonology the two high front vowels /i/ and /ü/ were merged, simplifying the six-vowel system to the five-vowel system of Modern Greek. In the morphology the frequent misuse of the dative case of nouns shows that it went out of use in the spoken language, and the infinitive was replaced by various periphrastic constructions. (Periphrastic constructions involve the use of function words and auxiliaries.) In the earlier period numerous words (mostly Latin) were imported: the chronicler Malalas has (in their modern form) pórta ‘door,’ kámbos ‘plain,’ saíta ‘arrow,’ paláti ‘palace,’ spíti ‘house’ (from hospitium), and hundreds of other borrowings, not all of which have survived. The later period is characterized by the richness of its compound words, usually from native roots. Some of these, such as the compounds in which a modifying noun precedes its head noun, continued ancient patterns (thalassóvrakhi ‘sea rock,’ vunópulo ‘mountain lad’); coordinative compounds of the type common in Modern Greek, though rare in earlier periods, are also found (aristódhipnon ‘lunch and dinner,’ compare Modern Greek andróyino ‘man and wife,’ makheropíruna ‘knives and forks’). Semantic shift was another source of innovation: álogho ‘horse’ previously meant ‘irrational’; skiázome ‘I fear’ earlier meant ‘I am in shadow’; and (u)dhén ‘not’ meant, in Classical Greek, ‘nothing.’