the device of modulation by which one passes from one key to another. Still it remained difficult to get far away from the key in which one started out, because of the manner of tuning, which made only a few keys available at once; but J. S. Bach, modifying the system of tuning to what is called equal temperament,[6] which opens the doors simultaneously to the entire twelve keys, emancipated music entirely from the restrictions of the ecclesiastical modes, and in his great work, “The Well-[or Equally-] Tempered Clavichord,” demonstrated practically the use of all the twelve keys as an intimate and compact family. By his time the principle of tonality was firmly established. A second principle vital to modern music is that of “thematic development.” By this is meant, first, the existence in the music of certain salient, easily recognizable groups of tones, called motifs, subjects, or themes, which are presented to the hearer at the outset, and impressed upon him by their unique individuality of cut; and second, that subsequent elaboration of these themes, in varied but still recognizable forms, which corresponds closely with the process by which an essayist develops an idea, a mathematician proves a theorem, or a preacher elucidates a text. It is interesting to note that the German word “Satz,” often used by musicians to mean “a theme,” signifies primarily a thesis or proposition in logic, while “Durchführung,” used to describe the development of the theme, means primarily a leading-through or bringing to an issue. Thus the process of thematic development in music is much like any other process of intellectual statement and proof. Now it is evident that this process, which is indispensable to all the higher intellectual forms of music, requires in the first place definite, concise, and memorable themes, since it is impossible to discuss what one fails to grasp, or after grasping, forgets. As the proverb says, the preparer of a ragout of hare must “first catch his hare.” Similarly musicians, before they could make their music logical, had to catch their themes. But as musical material up to the time of Palestrina never was definite or memorable, the first requisite of thematic music was some principle by which themes could be defined. This principle was found in the time-measurement of tones. So soon as a group of tones were placed in measured relations of duration to one another, an individual theme emerged, and could be elaborated. The second great conquest of modern music, then, was the conquest of the definite theme or motif, strictly measured in time, and of those devices by which it could be developed in an extended and logical discourse.