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.
- On the seventh note (the subtonic) of the minor scale (the Aeolian scale). That is, if a dominant seventh chord is composed starting with the seventh note on the minor scale, then all notes in that chord will also be on the minor scale.
- On the fifth note (the dominant) of the major scale (the Ionian scale).
- On the fifth note (the dominant) of the harmonic minor scale.
- On the fourth note (the subdominant) and the fifth note (the dominant) of the melodic ascending minor scale.
- On the fifth note (the dominant) and the sixth note (the submediant) of the altered scale.
- On the first note (the tonic) of the Spanish gypsy scale.