MODULATION. – This term essentially denotes the transition from one key to another.
The means by which such a transition can be effected vary widely and differ in individual cases, depending on the number of chords employed, the character one wishes to impart to the modulation, its duration and complexity, its style and expressive value; more technically, it depends on the dissonances involved, the greater or lesser prevalence of diatonicism or chromaticism, and the various procedures by which the notes of the arrival key are introduced, either successively or simultaneously, gradually or abruptly eliminating those of the departure key, with or without the use of suspensions, symmetrical patterns, rhythmic and melodic elaborations. Therefore, modulations are commonly distinguished, according to their duration, into momentary, temporary, and definitive. Momentary modulations abandon the arrival key immediately after reaching it; temporary modulations remain in it for a more or less extended time without eliminating the need to return to the principal key; definitive modulations replace the principal key with another principal key. Modulations can be immediate (transitions); yet it is arguable that true modulation is always mediated, consisting in finding one or more connecting links between two keys. As for the relationship between keys, modulations are distinguished into close and remote. Close keys are those having in their key signature one more or one fewer accidental than the key taken as a basis. One can modulate to a close key by momentarily touching remote keys. Thus, direct and indirect modulations are further distinguished. For example: from C major to A-flat major through A minor: direct modulation; from C major to A-flat major through E major, G minor, and E-flat major: indirect modulation, i.e., to a close key through remote keys.
In the works of great composers, modulation is never an end in itself but always has an intimate relationship with the structure of the piece, or a particular expressive, descriptive, or allusive character, justified by the words in the case of music set to a given text. In the accompanied recitatives of the great Italian composers of the 19th century, there are frequent examples of modulations that closely adhere, with the most felicitous effect, to the emotional states of a given character; in the masters of the lied, the art of modulation according to the suggestions of the poetic text is exquisite and highly refined. In symphonic music, there are modulations necessary for the very construction of the piece, i.e., modulations dictated by the musical form, which carries within itself its own tonal laws. Thus, for example, the first movement of a sonata or quartet is structured around two principal symphonic ideas, the second of which appears in the key of the major dominant or the so-called relative major (and sometimes in other subordinate keys).
The evolution of musical language in the past fifty years has greatly enriched the technique of modulation, both in the polytonal realm (simultaneous modulations) and in the modal realm (unimodal, intermodal, and polymodal modulations).