APPROVED ; This dissertation establishes the techniques employed in the organ accompaniment of plainchant, determines whether consensus was reached on the adoption of such techniques, and illustrates the musical and commercial factors in the publishing of accompaniment books and theoretical manuals. Publications in some twenty languages provide the basis for the discussion which is illustrated by quoted music examples. These bear witness to a highly adaptive set of musical techniques that responded to changes in fashion and discoveries in music history. The discussion is further illustrated by referencing a rich vein of letters and archival material -here being brought to light for the first time- that show how ideas passed between theorist and practitioner and how methods, manuals, theory and practice transcended international and religious boundaries, leaving an enduring legacy that may still be felt in sacred music today. A new story is told, largely in chronological order, one of musical idealism, political wrangling and the commercial shrewdness of musicians and publishers who responded to the demands of the market. The theory and practice of chant accompaniment at Solesmes serves as a particular focus since the importance of accompaniments in the chant restoration movement had not been considered prior to the present redaction.