The Horologion is a liturgical book used mainly by cantors or church readers which contains the services or offices to be read (chanted) during the different hours of each day. Its name is derived from the Greek word hora, which means “hour” or “time,” and thus means “Book of Hours.” In Russian this book is called a Chasoslov.

The Horologion contains the services for Vespers, Compline, Nocturns, and Matins, as well as the First, Third, Sixth, and Ninth Hours themselves. The Great Horologion also often contains morning prayers, evening prayers, the All-Night Vigil, and various Troparia and Kontakia hymns.