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As early as in the Hellenistic period, there was a tendency towards a state of diglossia between the Attic literary language and the constantly developing vernacular Koiné. By late antiquity, the gap had become impossible to ignore. In the Byzantine era, written Greek manifested itself in a whole spectrum of divergent registers, all of which were consciously archaic in comparison with the contemporary spoken vernacular, but in different degrees.
They ranged from a moderately archaic style employed for most every-day writing and based mostly on the written Koiné of the Bible and early Christian literature, to a highly artificial learned style, employed by authors with higher literary ambitions and closely imitating the model of classical Attic, in continuation of the movement of Atticism in late antiquity. At the same time, the spoken vernacular language developed on the basis of earlier spoken Koiné, and reached a stage that in many ways resembles present-day Modern Greek in terms of grammar and phonology by the turn of the first millennium AD. Written literature reflecting this demotic Greek begins to appear around 1100.
Among the preserved literature in the Attic literary language, various forms of historiography take a prominent place. They comprise chronicles as well as classicist, contemporary works of historiography, theological documents, and saints' lives. Poetry can be found in the form of hymns and ecclesiastical poetry. Many of the Byzantine emperors were active writers themselves and wrote chronicles or works on the running of the Byzantine state and strategic or philological works.
Furthermore, letters, legal texts, and numerous registers and lists in Medieval Greek exist. Concessions to spoken Greek can be found in literature in the following examples: John Malalas's Chronography from the 6th century, the Chronicle of Theophanes the Confessor (9th century) and the works of Emperor Constantine VII Porphyrogenitus (mid-10th century). These are influenced by the vernacular language of their time in choice of words and idiom, but largely follow the models of written Koine in their morphology and syntax.
The spoken form of Greek was called glossa dēmodēs (γλῶσσα δημώδης, "vernacular Greek"), haploellinikē (ἁπλοελληνική, "basic Greek"), kathomilimenē (καθωμιλημένη, ‘spoken’) or Romaiikē (Ῥωμαϊική, "Roman language"). Before the 13th century, examples of texts written in vernacular Greek, are very rare. They are restricted to isolated passages of popular acclamations, sayings, and particularly common or untranslatable formulations which occasionally made their way into Greek literature. Since the end of the 11th century, vernacular Greek poems from the literary realm of Constantinople are documented.
The Digenes Akritas, a collection of heroic sagas from the 12th century that was later collated in a verse epic, was the first literary work completely written in the vernacular. The Greek vernacular verse epic appeared in the 12th century, around the time of the French romance novel, almost as a backlash to the Attic renaissance during the dynasty of the Komnenoi in works like Psellos’s Choreography (in the middle of the 11th century) or the Alexiad, the biography of Emperor Alexios I Komnenos written by his daughter Anna Komnena about a century later. In fifteen-syllable blank verse (versus politicus), the Digenes Akritas deals with both ancient and medieval heroic sagas, but also with stories of animals and plants. The Chronicle of the Morea, a verse chronicle from the 14th century, is unique. It has also been preserved in French, Italian and Aragonese versions, and covers the history of French feudalism on the Peloponnese.
The earliest evidence of prose vernacular Greek exists in some documents from southern Italy written in the tenth century. Later prose literature consists of statute books, chronicles and fragments of religious, historical and medical works. The dualism of literary language and vernacular was to persist until well into the 20th century, when the Greek language question was decided in favor of the vernacular in 1976.