The harmonic seventh interval, also known as the septimal minor seventh, is one with an exact 7:4 ratio (about 969 cents). This is somewhat less than and is "sweeter in quality" than an "ordinary" minor seventh, which has a just-intonation ratio of either 16:9 or 9:5, or an equal-temperament ratio of 1000 cents.
The harmonic seventh chord is a major triad plus the above-mentioned harmonic seventh interval. 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).
The harmonic seventh differs from the augmented sixth by 224/225, or about a 1/3 of a comma. The harmonic seventh note is about a quarter-tone flatter than an equal tempered minor seventh. When this flatter seventh is used, the dominant seventh chord's "need to resolve" down a fifth is weak or non-existent. This chord is often used on the tonic (written as I7) and functions as a "fully resolved" final chord.
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Last updated on Saturday July 19, 2008 at 18:59:13 PDT (GMT -0700)
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