The harmonic seventh chord is a major triad plus the harmonic seventh interval (ratio of 7:4, about 968.826 cents). This interval is somewhat narrower (about 48.77 cents, a septimal quarter tone) and is "sweeter in quality" than an "ordinary" minor seventh, which has a just-intonation ratio of 9:5 (1017.596 cents), or an equal-temperament ratio of 1000 cents (25/6:1). Frequent use of this chord is one of the defining characteristics of blues and barbershop harmony; barbershoppers refer to it as "the barbershop seventh". Since barbershop music tends to be sung in just intonation, the barbershop seventh chord may be accurately termed a harmonic seventh chord. The harmonic seventh chord is also widely used in "blues flavored" music. As guitars, pianos, and other equal-temperament instruments cannot play this chord, it is frequently approximated by a dominant seventh. As a result it is often called a dominant seventh chord and written with the same symbols (such as the blues progression I7 - V7 - IV7).
An often heard example of the harmonic seventh chord is the last word of the modern addition to the song "Happy Birthday to You", with the lyrics, "and many more!" The harmony on the word "more" is typically sung as a harmonic seventh chord.
The alpha scale has, "excellent harmonic seventh chords...using the inversion of 7/4, i.e., 8/7." Play.
It is suggested that the harmonic seventh on the dominant not be used as a suspension, since this would create a mistuned fourth over the tonic. The harmonic seventh of G, F+, is lower than the perfect fourth over C, F♮, by Archytas' comma (27.25 cents).
Read more about Harmonic Seventh Chord: Barbershop Seventh
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