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 sequences: Midnight Office with the introductory prayer, Orthros with the First Hour, Third Hour, Sixth Hour, Ninth Hour, Vespers, and Compline (Great or Small).
As is known, all these sequences are contained in the Great Horologion. And depending on the festive or non-festive content of each day, Vespers and Orthros present variations and are supplemented with hymns from other liturgical books: the Paraklitiki contains the hymnography of each day of the week in the eight tones, the Menaion contains the fixed feasts for all days of the year, the Triodion and the Pentecostarion contain the movable feasts of the corresponding periods. And the Typikon is the guide for the performance of the divine services in various cases.
It is evident that the complete performance of the sequences 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, which contain the most essential elements, with the aim not to replace the regular liturgical books, but to serve individual prayer and, to some extent, the participation in the sequences in the church.