When a particular phase of music theory is omitted from the contents of a treatise or textbook on that subject, the omission can usually be ascribed to one of two causes. Either the omitted phase is of little importance or it is not used in the music of the period on which the treatise or textbook is based. It is the purpose of this research study to discuss a particular phase of music theory that has been omitted or avoided by numerous counterpoint and theory textbook authors. The material in the contents of this work is based on a discussion of the melodic use of the augmented second in the music of the eighteenth century.