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The most important means of gaining unity and coherence in a composition is to have it written in a clearly defined tonality, especially at the outset. This definite tonality is the "centre of gravity," so to speak, about which the whole composition revolves. If this tonal centre were uncertain or wandering, we should have a feeling of vagueness and perplexity which, except for special dramatic effect, is never found in works of the great composers. Thus we speak of a Symphony in C minor, of a Quartet in F major and of a Sonata in B-flat minor;[46] this foundation key being comparable to the basic color-scheme of a painting. There is also a particular aesthetic effect and color-appeal associated with each key; and the listener should train himself to be sensitive to the brilliance of such keys as D major and E major, the richness of B major, the dignity of E-flat major, the almost cloying sweetness of D-flat major and of G-flat major and the tragic depth of B minor and G minor. No piece, however, should remain for long in the same key; for music cuts so deeply into the consciousness that there would result an intolerable monotony.[47] Even in the simplest folk-songs, therefore, we often find manifested an instinct for those changes of tonal centre which are technically called "Modulations." All the keys founded on the twelve semitones of the chromatic scale are related—though in varying degrees of closeness; and in modern music, no matter how complex the modulations often sound, we may be sure that the composer plans them as carefully as the painter adjusts his color-scheme. For definite acoustical[48] and harmonic reasons, however, the keys most closely related to a given tonal centre are those situated a perfect fifth above—the Dominant; a perfect fifth below—the Subdominant; and the Relative Minor, the key-note of which is a minor third below, e.g., A minor in relation to C major, C minor to E-flat major. The relative minors of the Dominant and Subdominant also bear a close relationship to a given tonic; and into these five keys is made a large majority of the modulations in any piece of music.[49]
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Beginning with Beethoven, a modulation into what are known as the mediant keys became frequent; and is, in fact, a favorite change in all modern music—the mediant keys being those situated half-way between a Tonic and Dominant or a Tonic and Subdominant, e.g.
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Anyone at all familiar with Beethoven's style will remember how often his second theme, instead of following the more conventional line of dominant relationship, is in a mediant key. Good examples may be found in the first movement of the Waldstein Sonata and in the first and last movements of the 8th Symphony. A little thought will make clear that the relationships just set forth include nearly all the possible ones save those of 2nds and 7ths. Even into these apparently distant keys, e.g., to D-flat major or to B major from C major, modulations may easily be made by means of the "enharmonic"[50] relationship found in that frequently used modern chord—the Augmented Sixth, e.g.
