Gregorian chant is monophonic: a single line of melody, sung in unison and unaccompanied, its free rhythm rising and falling with the words rather than a beat. It carries the texts of the Mass and the Liturgy of the Hours — above all the Psalms — and is sung in Latin, the words always governing the music. Tradition credits its ordering to Pope Gregory the Great (d. 604), though the repertory took its lasting form in the Frankish lands in the centuries after him.
The chant developed the West's system of musical notation — the square 'neumes' on a staff that still print it today — and so stands near the headwaters of Western music itself. But its purpose was never performance. The chant is prayer made audible: the melody serves the sacred text, lifting it, slowing it, letting it be heard and held. It is, in the old phrase, 'sung prayer.'
Gregorian chant remains the Roman Catholic Church's own proper music, honored anew whenever the liturgy is renewed, and it has drawn wide love far beyond the cloister and the Catholic world. The Christian East has its own great chant traditions — Byzantine, Syriac, Znamenny — each a parallel witness to the same conviction: that the holiest words are best not merely spoken but sung.