<P> Gregorian chant evolved to fulfill various functions in the Roman Catholic liturgy . Broadly speaking, liturgical recitatives are used for texts intoned by deacons or priests . Antiphonal chants accompany liturgical actions: the entrance of the officiant, the collection of offerings, and the distribution of sanctified bread and wine . Responsorial chants expand on readings and lessons . </P> <P> The non-psalmodic chants, including the Ordinary of the Mass, sequences, and hymns, were originally intended for congregational singing . The structure of their texts largely defines their musical style . In sequences, the same melodic phrase is repeated in each couplet . The strophic texts of hymns use the same syllabic melody for each stanza . </P> <P> Early plainchant, like much of Western music, is believed to have been distinguished by the use of the diatonic scale . Modal theory, which postdates the composition of the core chant repertory, arises from a synthesis of two very different traditions: the speculative tradition of numerical ratios and species inherited from ancient Greece and a second tradition rooted in the practical art of cantus . The earliest writings that deal with both theory and practice include the Enchiriadis group of treatises, which circulated in the late ninth century and possibly have their roots in an earlier, oral tradition . In contrast to the ancient Greek system of tetrachords (a collection of four continuous notes) that descend by two tones and a semitone, the Enchiriadis writings base their tone - system on a tetrachord that corresponds to the four finals of chant, D, E, F, and G . The disjunct tetrachords in the Enchiriadis system have been the subject of much speculation, because they do not correspond to the diatonic framework that became the standard Medieval scale (for example, there is a high F#, a note not recognized by later Medieval writers). A diatonic scale with a chromatically alterable b / b - flat was first described by Hucbald, who adopted the tetrachord of the finals (D, E, F, G) and constructed the rest of the system following the model of the Greek Greater and Lesser Perfect Systems . These were the first steps in forging a theoretical tradition that corresponded to chant . </P> <P> Around 1025, Guido d'Arezzo revolutionized Western music with the development of the gamut, in which pitches in the singing range were organized into overlapping hexachords . Hexachords could be built on C (the natural hexachord, C-D-E ^ F-G-A), F (the soft hexachord, using a B - flat, F-G-A ^ Bb - C-D), or G (the hard hexachord, using a B - natural, G-A-B ^ C-D-E). The B - flat was an integral part of the system of hexachords rather than an accidental . The use of notes outside of this collection was described as musica ficta . </P>

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