The chief problem of composition, in any style from ancient times to the present, is the creation of a form, or structure in which the principles of unity and contrast operate in some kind of equilibrium. The listener enters into this process by the use of his powers of recognition and of memory.
In purely melodic, modal music the form often derives from the inner logic of the melody in terms of the important notes and melodic patterns of the mode. In polyphonic music before the common practice period, musical form depended partly on the unity achieved by setting a piece in a given mode, partly on the use of musical themes, and partly on creating harmonic movement and tension toward stopping points, or cadences.
During the common practice period—from Bach to Debussy—much of the creation of musical form took place through the organization of harmonies into keys and relationships between keys.
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Thus, the sonata forms of the 18th and early 19th centuries depended as much on the statement of a key, the movement to other key areas, and the eventual return to the same key as they did on themes and other melodic devices. The composer was likely, of course, to employ the two principles of melody and harmony simultaneously; the return to the tonic key late in the course of a movement was usually reinforced by a restatement of the initial themes. In certain works of Haydn and Mozart, the listener was often thrown off course purposely by the premature return of initial themes in an unexpected key; such devices served further to enhance the drama of the genuine recapitulation, or return to the main key.
By the 19th century, however, the power of harmony to suggest clear formal structures was greatly undermined by freer use of dissonance, which broke down the clarity with which a key was defined. Other customary procedures were also abandoned. Many of Schumann’s songs do not return to the tonic, or home key, for the final cadence. The extended length of Wagner’s music dramas, and their wide-ranging modulation, make it impossible to regard key as a unifying force. Mahler’s Ninth Symphony, the first movement of which is in D major, ends with a movement in D flat major, and since the symphony lasts nearly 90 minutes, there seemed to Mahler to be no reason to pay any closer lip service to classical practices of unity of key. Such necessities, by the time of this symphony, had vanished from the musical language.
