At the end of the Heian period, it seems to have often been used in spoken language same as 'desu-masu style' (a formal and polite form of Japanese conversational style) (There are many examples of usage in the narrative part of Heike Monogatari (The tale of the Heike)).
平安末期には現代語の「ですます体」のように口語で盛んに用いられたらしい（平家物語の語りの部分に多くの用例がある）。