Of all the seventh chords, perhaps the most important to understand is the major triad with a minor seventh, also called the dominant seventh chord. Called the Dominant Seventh because its intervallic relationships occur naturally in the seventh chord built on the dominant scale degree of a given key (e.g., G7 in the key of C major), the dominant seventh chord was the first to begin to appear regularly in Western music. The dominant seventh chord is useful to composers because it is a major chord with a very strong sound, that also includes a tritone between the third and seventh of the chord. In a diatonic context, the third of the chord is the leading-tone of the scale, which has a strong tendency to pull towards the tonal center, or root note, of the key. This, in combination with the strength of root movement by fifth, and the natural resolution of the dominant triad to the tonic triad, creates a resolution with which to end a piece or a section of a piece. Because of this original usage, it also quickly became an easy way to trick the listener's ear with a deceptive cadence. The most important usage, though, is the way that the introduction of a non-diatonic dominant seventh chord (sometimes called a chromatic seventh) which is borrowed from another key, can allow the composer to modulate to that other key. This technique is extremely common, particularly since the classical period, and has led to further innovative uses of the dominant seventh chord such as secondary dominant, extended dominant, and substitute dominant chords. |
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