When once composers had grasped the possibilities of structure by means of harmony, they quickly proceeded to work them out in the large, as applied to a complete musical form. They began to organize whole pieces by means of a grouping or ordered antithesis of different harmonic centres. Working without models and in the dark, they made many false starts and wrong moves, they tried many hybrid and unstable forms; but eventually, in the course of years of experiment, they developed two great types of structure, based on fundamental principles, and embodied, with unimportant minor modifications, in almost all the suite-movements of the seventeenth and of later centuries. The first of these two great general types of structure, called Binary Form, contained two distinct members or sections; the second, called Ternary Form, contained three sections.
The essential principle of binary form is the simplest conceivable. Every piece in binary form may be likened to a journey to a neighboring place, followed by a return home. “The King of France, with forty thousand men, marched up the hill, and then marched down again.” In the case of binary form, the king of France is the subject or theme of the piece; the forty thousand men are the variations or developments on this subject that are worked out as the piece proceeds; the hill is the progress from the tonic key to the contrasted tonal centre, generally the dominant, or, if the piece is in a minor key, its relative major; and the march down again is the return to the home key. More specifically, the first section begins with the announcement of the theme in the tonic key, and proceeds to ring changes upon it, meanwhile modulating to the contrasted key and ending with a firm and memorable cadence there. At this point the second section begins, with the theme as at first, but in the new instead of the original key; the modulation is reversed, the original key re-entered, and the same cadence already heard repeated, but now even more firmly, and with the added finality of the home key. The device is simplicity itself, yet it admits a surprising variety of detail within its perfectly obvious and satisfying unity of ultimate effect. Most of Bach’s allemandes, courantes, airs, sarabandes, and gigues, are executed in binary form.