Although Katharevousa was introduced relatively recently, the concept of a conservative literary version of Greek had been present since the times of Koine Greek. There had always existed a tendency towards a state of diglossia between the Attic literary language and the constantly developing spoken Koine, which eventually evolved into Demotic Greek. Medieval Greek texts and documents of the Byzantine Empire were almost always written in conservative literary Greek. Examples of texts written in vernacular Greek prior to the 13th century are very rare. It can be argued that the establishment of Katharevousa was an official declaration and standardization of the conservative form of Greek, which had already existed in one way or another.