One theory of the origin, development and structure of the chant repertoire is that the thousands of individual pieces in it arose from the adaptation of new texts to a small number of melodies, or melodic ‘themes’. First put forward by Gevaert, this theory, which will be referred to here as the thematic theory, has, in different forms, been the basis of studies of the transmission of antiphons, and is now being applied to the repertories of Old-Roman and Ambrosian chant. The main demonstration of the validity of the theory must take place, of course, in studies of the repertoire itself; we shall be convinced if musical and liturgical evidence supports the idea of adaptation and suggests a chronological development. It is nevertheless important to examine music-historical witnesses ancillary to the chant repertoire, for it is often the case that related documents reveal something unexpected about the main repertoire, or serve as a means of confirming points for which evidence is lacking.
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