How did Byzantine Greek differ from the spoken Greek of its time?

Byzantine Greek was characterized by a state of diglossia, where a significant rift existed between the written and spoken forms of the language.1

The Written Language

"Byzantine Greek" typically refers to the archaizing, formal style used in administration, literature, and the Greek Orthodox church. This style was firmly rooted in the Atticist tradition, which consciously sought to emulate the language of classical antiquity. Because this written form was taught through a rigid educational system that prioritized rhetoric, it remained largely static and far removed from the everyday speech of the population.21

The Spoken Language

In contrast, the spoken language continued to evolve naturally, moving toward the structure of Modern Greek. Key differences included:32

• Phonology: The complex vowel system of earlier periods simplified, notably merging high front vowels into a five-vowel system.2
• Morphology: The dative case fell out of use, and the infinitive was replaced by periphrastic constructions.2
• Vocabulary: The spoken language incorporated numerous loanwords, particularly from Latin (e.g., spíti for "house"), and underwent significant semantic shifts, such as álogho changing from "irrational" to "horse".2

While some later chronicles began to reflect these colloquial developments, the prestige of the archaizing written style remained dominant for serious literature until the fall of Constantinople in 1453.21