Gregorian chant is the central tradition of Western plainchant, a form of monophonic, unaccompanied sacred song in Latin 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 it arose from a later Carolingian synthesis of the Old Roman chant and Gallican chant.

In music theory, the term mode or modus is used in a number of distinct senses, depending on context.