Cherubikon

The Cherubikon (Greek: χερουβικόν) is the usual Cherubic Hymn (Greek: χερουβικὸς ὕμνος, Church Slavonic ) sung at the Great Entrance of the Byzantine liturgy. The hymn symbolically incorporates those present at the liturgy into the presence of the angels gathered around God's throne.
History.
Origin.
The cherubikon was added as a troparion to the Divine Liturgy under Emperor Justin II (565 – 578) when a separation of the room where the gifts are prepared from the room where they are consecrated made it necessary that the Liturgy of the Faithful, from which those not baptised had been excluded, start with a procession. This procession is known as the Great Entrance, because the celebrants have to enter the choir by the altar screen, later replaced by the iconostasis. The chant genre "offertorium" in traditions of Western plainchant was basically a copy of the Byzantine custom, but there it was a proper mass chant which changed regularly.
Although its liturgical concept already existed by the end of the 4th century, the cherubikon itself was created 200 years later. The Great Entrance as a ritual act is needed for a procession with the Gifts while simultaneous prayers and ritual acts are performed by the clergy. As the processional troparion, the cherubikon has to bridge the long way between prothesis, a room outside the apsis, and the sanctuary which had been separated by changes in sacred architecture under Emperor Justin II. The cherubikon is divided into several parts. The first part is sung before the celebrant begins his prayers, there were one or two simultaneous parts, and they all followed like a gradual ascent in different steps within the Great Entrance. Verses 2-5 were sung by a soloist called monophonaris from the ambo. The conclusion with the last words of verse 5 and the allelouiarion are sung in dialogue with the domestikos and the monophonaris.
Liturgical use.
Concerning the text of the processional troparion which was ascribed to Justin II, it is not entirely clear, whether "thrice-holy hymn" did refer to the "Sanctus" of the Anaphora or to another hymn of the 5th century known as the "trisagion" in Constantinople, but also in other liturgical traditions like the Latin Gallican and Milanese rites. 
Concerning the old custom of Constantinople, the trisagion was used as a troparion of the third antiphonon at the beginning of the divine liturgy as well as of hesperinos.
In the West, there were liturgical customs in Spain and France, where the trisagion replaced the great doxology during the Holy Mass on lesser feasts.
The troparion of the great entrance (at the beginning of the second part of the divine liturgy which excluded the catechumens) was also the prototype of the genre offertorium in Western plainchant, although its text only appears in the particular custom of the Missa graeca celebrated on Pentecost and during the patronal feast of the Royal Abbey of Saint Denis, after the latter's vita became associated with Pseudo-Dionysios Areopagites. According to the local bilingual custom the hymn was sung both in Greek and in Latin translation.
Today, the separation of the prothesis is part of the early history of the Constantinopolitan rite ("akolouthia asmatike"). With respect to the Constantinopolitan customs there are many different local customs in Orthodox communities all over the world and there are urban and monastic choir traditions in different languages into which the cherubikon has been translated.
Exegetic tradition of Isaiah.
The trisagion or thrice-holy hymn which was mentioned by John Chrysostom, could only refer to the Sanctus of the Anaphora taken from the Old Testament, from the book of the prophet Isaiah in particular (6:1-3):
[1] And it came to pass in the year in which king Ozias died, that I saw the Lord sitting on a high and exalted throne, and the house was full of his glory. [2] And seraphs stood round about him, each one had six wings, and with two they covered their face, and with two they covered their feet, and with two they flew. [3] And one cried to the other, and they said "Holy, holy, holy is the Lord of hosts! The whole earth is full of His glory!"
In a homily John Chrysostom interpreted Isaiah and the chant of the divine liturgy in general (neither the cherubikon nor the trisagion existed in his time) as an analogue act which connected the community with the eternal angelic choirs:
On high, the armies of angels give glory; below, men, standing in church forming a choir, emulate the same doxologies. Above, the Seraphim declaim the thrice-holy hymn; below, the multitude of men sends up the same. A common festival of the heavenly and the earthly is celebrated together; one Eucharist, one exultation, one joyful choir.
The anti-cherubika.
The cherubikon belongs to the ordinary mass chant of the divine liturgy ascribed to John Chrysostom, because it has to be sung during the year cycle, however, it is sometimes substituted by other troparia, the so-called "anti-cherubika", when other formularies of the divine liturgy are celebrated. 
On Holy Thursday, for example, the cherubikon was, and still is, replaced by the troparion "At your mystical supper" (Τοῦ δείπνου σου τοῦ μυστικοῦ) according to the liturgy of Saint Basil, while during the Liturgy of the Presanctified the troparion "Now the powers of the heavens" (Νῦν αἱ δυνάμεις τῶν οὐρανῶν) was sung, and the celebration of "Prote Anastasis" (Holy Saturday) uses the troparion from the Liturgy of St. James, "Let All Mortal Flesh Keep Silence" (Σιγησάτω πᾶσα σὰρξ βροτεία). The latter troparion is also used occasionally at the consecration of a church.
Text.
In the current traditions of Orthodox chant, its Greek text is not only sung in older translations such as the one in Old Church Slavonic or in Georgian, but also in Romanian and other modern languages.
In the Greek text, the introductory clauses are participial, and the first person plural becomes apparent only with the verb ἀποθώμεθα "let us lay aside". The Slavonic translation mirrors this closely, while most other translations introduce a finite verb in the first person plural already in the first line (Latin "imitamur",
Georgian "vemsgavsebit", 
Romanian "închipuim" "we imitate, represent").
The notated chant sources.
Due to the destruction of Byzantine music manuscripts, especially after 1204, when Western crusaders expelled the traditional cathedral rite from Constantinople, the chant of the cherubikon appears quite late in the musical notation of the monastic reformers, within liturgical manuscripts not before the late 12th century. This explains the paradox, why the earliest notated sources which have survived until now, are of Carolingian origin. They document the Latin reception of the cherubikon, where it is regarded as the earliest prototype of the mass chant genre offertorium, although there is no real procession of the gifts.
The Latin cherubikon of the "Missa greca".
The oldest source survived is a sacramentary ("Hadrianum") with the so-called "Missa greca" which was written at or for the liturgical use at a Stift of canonesses (Essen near Aachen). The transliterated cherubikon in the center like the main parts of the Missa greca were notated with paleofrankish neumes between the text lines. Paleofrankish neumes are adiastematic and no manuscripts with the Latin cherubikon have survived in diastematic neumes. Nevertheless, it is supposed to be a melos of an E mode like the earliest Byzantine cherubika which have the main intonation of .
In this particular copy of the Hadrianum the "Missa greca" was obviously intended as proper mass chant for Pentecost, because the cherubikon was classified as "offertorium" and followed by the Greek Sanctus, the convention of the divine liturgy, and finally by the communio "Factus est repente", the proper chant of Pentecost. Other manuscripts belonged to the Abbey Saint-Denis, where the Missa greca was celebrated during Pentecost and in honour of the patron within the festal week (octave) dedicated to him. Sacramentaries without musical notation transliterated the Greek text of the cherubikon into Latin characters, while the books of Saint-Denis with musical notation translated the text of the troparion into Latin. Only the Hadrianum of Essen or Korvey provided the Greek text with notation and served obviously to prepare cantors who did not know Greek very well.
The cherubikon asmatikon.
In the tradition of the cathedral rite of the Hagia Sophia, there was only one melody in the E mode (), which has survived in the Asmatika (choir books) and, in a complete form, as "cherouvikon asmatikon" in the books Akolouthiai of the 14th and 15th century.
In this later elaboration, the domestikos, leader of the right choir, sings an intonation, and the right choir performs the beginning until μυστικῶς. Then the domestikos intervenes with a kalopismos over the last syllable το—το and a teretismos (τε—ρι—ρεμ). The choir concludes the kolon with the last word εἰκονίζοντες. The left choir is replaced by a soloist, called "Monophonaris" (μονοφωνάρις), presumably the lampadarios or leader of the left choir. He sings the rest of the text from an ambo. Then the allelouia (ἀλληλούϊα) is performed with a long final teretismos by the choir and the domestikos.
The earlier asmatika of the 13th century only contain those parts sung by the choir and the domestikos. These asmatic versions of the cherubikon are not identical, but composed realizations, sometimes even the name of the cantor was indicated. Only one manuscript, a 14th-century anthology of the asma, has survived in the collection of the of Messina (I-ME Cod. mess. gr. 161) with the part of the psaltikon. It provides a performance of the monophonaris together with acclamations or antiphona in honour of the Sicilian King Frederick II and can be dated back to his time.
The cherubikon palatinon.
Another shorter version, composed in the without any teretismoi, inserted sections with abstract syllables, was still performed during celebrations of the imperial court of Constantinople by the choir during the 14th century. A longer elaboration of the cherubikon palatinon attributed to "John Koukouzeles" was transcribed and printed in the chant books used by protopsaltes today.
Papadic cherubikon cycles.
Today the common practice is to perform the cherubikon according to the echos of the week (octoechos). One of the earliest sources with an octoechos cycle is an Akolouthiai manuscript by Manuel Chrysaphes (GR-AOi Ms. 1120) written in 1458. He had composed and written down an own cycle of 8 cherubika in the papadic melos of the octoechos.
Until the present day the protopsaltes at the Patriarchate of Constantinople are expected to contribute their own realization of the papadic cycles. Because the length of the cherubikon was originally adapted to the ritual procession, the transcriptions of the print editions according to the New Method distinct between three cycles. A short one for the week days (since the divine liturgy became a daily service), a longer one for Sundays, and an elaborated one for festival occasions, when a bishop or abbot joined the procession.