Liber usualis

The Liber usualis is a liturgical book containing Gregorian chant, which is monophonic, rhythmically free liturgical music of the Roman Catholic Church. It is used to accompany the texts of the mass and the canonical hours (divine office). Gregorian chant is named after St. Gregory I, who collected and codified it during his papacy (590–604). Charlemagne later imposed it on his kingdom, leading to an assimilation with the existing Gallican chant tradition.12

The book includes settings for the Ordinary of the mass (texts that remain the same for every mass, such as the Kyrie, Gloria, Credo, Sanctus, and Agnus Dei) and the Proper of the mass (texts that vary for each mass, like the Introit, Gradual, Alleluia, and Communion). These chants evolved over centuries, with early forms being plainchant and later incorporating more complex musical styles.23