Enharmonicism steps to the fore only occasionally in 18th-century music. Indeed, over the past two centuries, it has been commonly assumed that it was invoked only when a special affect demanded it (as in the much-discussed "Dance of the Furies" from Rameau's Hippolyte et Aricie). But a survey of 18th-century music refutes this perception and reveals that the enharmonicism of the 18th century can be broadly defined as belonging to one of two categories: simultaneous or immediate enharmonicism, and retrospective enharmonicism. Most early 18th-century examples restrict their usage to the simultaneous/immediate type, which consists of reinterpretations of enharmonic pivot chords. Retrospective enharmonicism, on the other hand, is less common than immediate enharmonicism but is remarkable because it presages the expansion of the diatonic tonal system into the chromatic tonal system of the 19th century. Retrospective enharmonicism does not involve the reinterpretation of an enharmonic pivot chord, nor is a reinterpretation perceived at any one point; it becomes clear only in retrospect that one must have occurred. Rather than a negation of some resolution tendency, as happens in the reinterpretation of a dominant seventh as an augmented sixth, there is a (typically large-scale) trajectory away from some tonic which is eventually regained through the enharmonic door. Some note or chord is respelled as its enharmonic equivalent, but without any aural clue. Drastic key changes of the sort typically encountered in instances of retrospective enharmonicism are for the most part proscribed in the writings of such composers and theorists as Rameau, Kirnberger, Koch, Heinichen, and Vogler, all of whom wrote in detail about staying within an orbit of closely related keys and rarely going directly from one key to another too far away. Nevertheless, this type of enharmonicism was a recognized compositional resource which, though used relatively infrequently in the 18th century, came to occupy a more central place in the realm of available compositional techniques in the 19th century.
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