Neume in the context of Kyrie

Gregorian chant is the central tradition of Western plainchant, a form of monophonic, unaccompanied sacred song in Latin (and occasionally Greek) of the Roman Catholic Church. Gregorian chant developed mainly in western and central Europe during the 9th and 10th centuries, with later additions and redactions. Although popular legend credits Pope Gregory I with inventing Gregorian chant, scholars believe that he only ordered a compilation of melodies throughout the whole Christian world, after having instructed his emissaries in the Schola Cantorum, where the neumatical notation was perfected, with the result of most of those melodies being a later Carolingian synthesis of the Old Roman chant and Gallican chant.

Gregorian chants were organized initially into four, then eight, and finally 12 modes. Typical melodic features include a characteristic ambitus, and also characteristic intervallic patterns relative to a referential mode final, incipits and cadences, the use of reciting tones at a particular distance from the final, around which the other notes of the melody revolve, and a vocabulary of musical motifs woven together through a process called centonization to create families of related chants. The scale patterns are organized against a background pattern formed of conjunct and disjunct tetrachords, producing a larger pitch system called the gamut. The chants can be sung by using six-note patterns called hexachords. Gregorian melodies are traditionally written using neumes, an early form of musical notation from which the modern four-line and five-line staff developed. Multi-voice elaborations of Gregorian chant, known as organum, were an early stage in the development of Western polyphony.

Euouae (/juː.ˈuː.iː/ yew-OO-ee; sometimes spelled Evovae) is an abbreviation used as a musical mnemonic in Latin psalters and other liturgical books of the Roman Rite. It stands for the syllables of the Latin words saeculorum Amen, taken from the Gloria Patri, a Christian doxology that concludes with the phrase in saecula saeculorum. Amen. The mnemonic is used to notate the variable melodic endings (differentiae) of psalm tones in Gregorian chant.

In some cases, the letters of Euouae may be further abbreviated to E—E. A few books of English chant (notably Burgess and Palmer's The Plainchant Gradual) make use of oioueae for the equivalent English phrase, "world without end. Amen".