Not much is known about the language of Macedonia during the Byzantine years , but it most probably followed the general trends of this period, which proved crucial for the history of the Greek language. Already in the Early Christian years the last traces of the ancient Macedonian dialect disappeared.
At the same time the first features of the modern Macedonian dialects appeared in 'koine', which had replaced the local dialect. The modern dialects were probably fully formed about the 10th century, although the existing written testimonia cannot prove it with certainty. These dialects were used only in speech, while the older form of 'koine', which differed progressively more from the spoken language, remained official language of the state.