Acolouthia (Greek: ἀκολουθία, "a following"; Church Slavonic: последование, romanized: posledovanie) in the Eastern Orthodox and Eastern Catholic 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 (which does not change daily).
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.
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).
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).
At an All-Night Vigil Great Vespers is combined with Matins and First Hour to form one continuous service.
It is a very abbreviated forma and consist of the Usual Beginning, after which the Reader says Psalm 103 while the priest silently reads the Prayers of Lamplighting (Lychnic).
Then the Phos Hilaron (a short hymn, praising God at the time of sunset), the prokeimenon, the aposticha, the Nunc dimittis, the trisagion, and the apolytikion.
Great Vespers is celebrated on Sundays and feast days, either separately or as part of the All-Night Vigil.
The chanters sing Psalm 103 (either in its entirety or a selection of verses), while the priest comes to stand on the ambon in front of the Holy Doors to recite the "Prayers of Lamp-lighting", after which he goes back inside the sanctuary.
Then the chanters begin "Lord, I Have Cried" with eight stichera (ten on Sundays), while the deacon performs a censing of the entire church.
After the prokeimenon (and, on feast days, readings from the Old Testament) the deacon recites the ektenias, and the priest says a prayer while all reverently bow their heads.
There the deacon recites the Litiy (an ektenia, invoking the names of many saints, to which the choir answers Kyrie Eleison many times).
It is composed of psalms, a doxology, troparion, the trisagion, the Lord's Prayer, the Kyrie Eleison repeated twelve times, and invitatory versicles, and Psalms 50, 69, and 162, which are followed by the greater doxology, the Creed, the trisagion, the Lord's Prayer, the troparion proper to the feast, the Kyrie Eleison repeated forty times, several invocations, and the long prayers of dismissal.
The Midnight Office is normally celebrated only in monasteries, except for the Paschal Vigil when it has a unique structure followed only on that day.
The Sequences supply for all the Acolouths of the Daily Cycle the material required for a particular Remembrance — that is to say, the material proper to a particular day of the week, a particular date of the year, a particular day in the liturgical seasons (for instance, during Great Lent or the period between Pascha (Easter) and Pentecost).