A dominant seventh chord is a chord composed of four notes, where the distance between the first and the second note is four semitones, the distance between the second note and the third note is three semitones, and the distance between the third note and the fourth note is three semitones.
The G dominant seventh chord, for example, is G7 and consists of the notes G, B, D, and F. There are four semitones between G and B. There are three semitones between B and D. There are three semitones between D and F.
The dominant seventh chord can also be described as composed of the major third, the perfect fifth, and the minor seventh of some scale. In the Mixolydian scale in G, for example, the third note is B. B is a major third, because the interval between the first note G and B is four semitones. The fifth note on the scale is D. It is a perfect fifth, as the interval between the first note G and D is seven semitones. The seventh note is F. It is a minor seventh, as the interval between the first note G and F is ten semitones.
The dominant seventh chord is a major chord with the added minor seventh.
Examples of scales with dominant seventh chords
The following are examples of where the dominant seventh chord occurs in common heptatonic scales.
Examples of relationships between the dominant seventh chord and other four note chords
Shifting the second note of the dominant seventh chord one semitone down produces a minor seventh chord (i.e., switching from a major third to a minor third).
Shifting the fourth note of the dominant seventh chord one semitone up produces a major seventh chord (i.e., switching from a minor seventh to a major seventh).