Acolouthia, (from the Greek: akoloutheo, "to follow"; Slavonic: posledovanie) in the Eastern Orthodox churches, signifies the arrangement of the Divine Services (Canonical Hours or Divine Office), perhaps because the parts are closely connected and follow in order. In a more restricted sense, the term "acolouth" refers to the fixed portion of the Office (that which does not change from day to day). The portions of the Office that are variable are called the Sequences. While the structure and history of the various forms of the Divine Office in the numerous ancient Christian rites is exceedingly rich, the following article will restrict itself to the practice as it evolved in the Eastern Roman (Byzantine) Empire.
The Office is composed of both musical and rhetorical elements, the first usually given in the musical mode or tone (echos), according to which the liturgical compositions are chanted. There are eight musical modes: four primary and four secondary (plagal). The rhetorical elements are seldom given in a normal speaking voice, but are "read" in a simple recitative.
As the early chanters rarely used texts set to musical notation, they learned by heart the words and music of some standard hymn, and this served as a model for other hymns of the same rhythm or meter. For example, in a Canon, the strophe or stanza of a standard hymn which indicates the melody of a composition is known as an irmos (eirmos, hirmos). An irmos is placed at the beginning of an Ode to introduce the melody to which it should be chanted, and to tie the theme of the Biblical Canticle on which it is based to the hymns of the Ode that follow (see Canon). A katabasia is the irmos that is sung at the end of an Ode by the choir (which descend from their seats (kathismata) and stand on the floor of the church to sing it). The katabasia winds down the Ode and returns it again to the theme of the Biblical Canticle.
The Divine Services are composed of a number of cycles, some large some small, all of which combine to form the services. These cycles are governed by rules prescribed in the Typikon. The basic cycle is the Daily Cycle, which is composed of the Acolouthia (distinct services) into which the Sequences for that day—which are taken from the other cycles—are inserted. The liturgical day begins at sunset, so Vespers is the first service of the day.
The individual Offices that make up the daily cycle of services are: are Vespers, Midnight Office, Orthros (Matins), the four Little Hours, and Apodeipnon (Compline). To this could be added the Typika, which is said on days on which there is no celebration of the Divine Liturgy (Eucharist).
Most of the Offices start with the Usual Beginning (a blessing by the priest, followed by the Trisagion and other prayers, ending with the Lord's Prayer and the call to worship: "O come, let us worship God our King..."), followed by one or several Psalms. The Psalter figures prominently in the Divine Services, and are found both in the fixed and the moveable portions of the services.