This dissertation is an analysis of two of the six sonatas for organ using rhetorical-musical prescriptions from seventeenth and eighteenth-century German theorists. It undertakes to examine the way in which lines are built by application of figurae, to observe the design of each of the six movements, and to draw conclusions concerning implications for performance based upon the use of figurae in specific contexts. The period source on melodic design and the ordering of an entire movement based upon principles of rhetoric is Johann Mattheson's Per volkommene Capelmeister (1739). Guidelines for categorization of figures derive from the twentieth-century writers Timothy Albrecht, George Buelow, Lena Jacobson, and Peter Williams. Chapter I provides justification for the rhetorical approach through a brief description of the rise of the process as applied to composition during the Baroque period by relating Bach's own familiarity with the terminology and processes of rhetorical prescription, and by describing the implications for performance in observing the sonatas from the rhetorical viewpoint. Chapter II deals with the process of composition by rhetorical prescription in (1) the invention of the subject and its figural decoration and (2) the elaboration of the subject through the sixpart discourse of an entire movement. Specific figures of decoration are defined through examples of their use within the context of the sonatas. Chapter III constitutes the analysis of the six sonata movements. Chapter IV reinforces the justification of this type of analysis. The figures, as aids for inflection and punctuation, affect decisions concerning articulation of events and assist in effecting convincing performance.