Based on the psalmic expression "seven times a day" (Psalm 118, 164), the tradition of the Church divided the prayer of the day and night into seven parts, distinguishing the services: Midnight Office with the introductory prayer, Matins with the First Hour, Third Hour, Sixth Hour, Ninth Hour, Vespers, Compline (great or small).
As is well known, all these services are contained in the Great Horologion. Depending on whether the content of each day is festive or not, Vespers and Matins present variations and are supplemented with hymns from other liturgical books: the Parakletike contains the hymns for each day of the week in the eight tones, the Menaion the fixed feasts for all the days of the year, the Triodion and the Pentecostarion the movable feasts of the respective periods. The Typikon serves as a guide for the performance of the sacred services in various cases.
It is evident that the complete performance of the services requires, in addition to knowledge, many books that cannot always be available. For this reason, various liturgical aids ("prayer books") have been compiled, small in volume, containing the most essential elements, not to replace the regular liturgical books, but to assist in individual prayer and, to some extent, in following the services in the church.