The Kyrie eleison, which means Lord have mercy, is a development to all liturgies both East and West. It appears in the West taken from the East around the 6th century and is not even found in the East before the 4th century. It originated in the liturgy at Antioch and Jerusalem as the response to litanies. Although it appears in scripture, this is not strictly just borrowing from the scriptural expression. In scripture is appears with an object (i.e. Lord have mercy on us/me). In any case it starts to be used in the Antiochene rite and from there it spreads to the rest of the Church.1
It seems that at some point the Roman liturgy had litanies, which were sung by deacons or other clerics.2 Pope St. Gregory I, defending the use of the Kyrie as no mere adaptation towards the liturgy of Constantinople, writes that at daily Mass the litanies were left out so that the people might dwell on the meaning of the Kyrie. He explains that “kyrie eleison” was sung and the people responded with the same text. 3 There was no fixed number of repetitions originally when the litanies were left out, the leader of the choir watching for the sign to end the invocations. Eventually it was settled into a nine-fold invocation. 4 Another difference is the Roman adaptation of singing “Christe eleison,” which is done nowhere else but in the West. Over time the litany apparently fell out of use except for special masses like Easter Vigil, Whitsunday, and ordination Masses. 5 This dwelling upon the words, which originally are intended as responses to the litany, caused the musical notion to have many long neumes (e.g. the last syllable of the last Kyrie is a good example of a stretched neume), which stretch the Kyrie out over a length of time. In the Middle Ages they began to add words (tropes, the adding of which is a convention called farcing) to the text. The Kyriale still reminds us of this practice by its convention of naming Mass settings: Kyriale Rex Genitor (Mass VI), Orbis factor (XI), etc. 6 Sometimes these farces were done using a mixture of Latin and Greek: "Deus creator omnium, tu Theos ymon nostri pie, eleyson" from the Missale Sarum. 7 The use of farced liturgical texts was abolished by the reform of Pope St. Pius V.
In the reform of Pope Paul VI, the nine-fold repetition was abrogated. In its place is a single versicle and response for each invocation. The letters ij mean repeat again (the Graduale Romanum has "bis." meaning the same thing). In former times with the ninefold repetition after the first invocation the letters iij would appear meaning repeat two more times and after the first Kyrie of the third invocation the letters ij for repeat again, the last Kyrie being the one sung with the longest neumes. Sometimes in the Missal of Paul VI it is altogether left out when either the Asperges is done or the form for general confession includes the response itself (Cf. Option C in the Penitential Rite). This has some basis in an older rubric at Rome where the Kyrie was left out if it had been said before in the Litany (Kyrie non dicitur propter Litaniam processionis, ubi dictum est Kyrie; Ord. Roman. XI, n. 63, as was done apparently up to the twelfth century).8
"As long as we children of Eve are constrained to remain in this vale of tears weeping and mourning," as we pray in the Salve Regina, "in exile and misery (in exsilio), no prayer is so necessary, none so befitting our condition as the Kyrie, this heartfelt appeal, this humble cry for mercy to the triune God, who is compassionate and merciful, long suffering and plenteous in mercy (Ps. 102:8)."9 "[I]n order that the plentitude of Divine Mercy may descend upon us, the cry of the Kyrie must proceed from a heart penetrated with a lively sense of its poverty and misery." 10 "The Kyrie is, moreover, a fitting preface to the Gloria; filled with joy and gratitude at the very thought of the graces and favors of our merciful God, we are impelled to bless His holy name. "The Kyrie eleison, - that cry for mercy which is to be found in every liturgy of East and West, - 'seems introduced as if to give grander effect to the outburst of joy and praise which succeeds it in the Gloria in excelsis; it is a deepening of our humiliation, that our triumph may be the better felt.'"11
1 The Mass: A Study of the Roman Liturgy, Rev. Dr. Adrian Fortescue, Loreto Publications, 2003: original printing 1912, p. 231.
2 Ibid., pp. 232-33.
3 Ibid., p. 234.
4 Ibid., pp. 236-37.
5 Ibid., p. 235.
6 Ibid., p. 238.
7 Ibid., p. 239, ft. 1 .
8 The Holy Sacrifice of the Mass; Dogmatically, Liturgically and Ascetically Explained, Rev Dr. Nicholas Gihr, 6th edition, B. Herder Book Co, 1924, p. 390.
9 Ibid., pp. 392 –93.
10 Ibid., p. 393.
11 Ibid., p. 392, quoted from Cardinal Wiseman, location not cited.