Of all the seventh chords, perhaps the most important to understand is the 'dominant seventh' a major triad with a minor seventh. It was the first seventh chord to appear regularly in western music. The name comes from the fact that it occurs naturally in the seventh chords built on the dominant fifth scale degree of a given major key.
Take for example the key of C major:
Dominant seventh in C majo.png
The note G is the dominant degree of C major (C, D, E, F, G, A, B, C) - the fifth note of the major scale whose first note is C (the dominant degree of any major key is always the fifth note). When we arrange the notes of the C major scale in ascending pitch and use only these notes to build a seventh chord, and we start with G (not C), then the resulting chord, containing the four notes GBDF and called G7, is a major triad, GBD, with a dominant (minor) seventh, F (that is, from G to F includes seven notes).

This basic dominant seventh chord is useful to composers because it contains both a major triad and the interval of a tritone. The major triad confers a very "strong" sound. The tritone is created by the cooccurrence of the third degree and seventh degree (e.g., in the G7 chord, the acoustic distance between B and F is a tritone). 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 (e.g. the third of G7, B, is the leading tone of the key of C). This, in combination with the strength of root movement and the natural resolution of the dominant triad to the tonic triad (e.g., from GBD to CEG in the key of C major), 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

However, the most important use of the dominant seventh chord in musical composition 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.

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