Septimal major third
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In music, the septimal major third, also called the supermajor third (by eg Helmholtz) and sometimes BP third is the musical interval exactly or approximately equal to a 9/7 ratio of frequencies. In terms of cents, it is 435 cents, a quartertone of size 36/35 sharper than a just major third of 5/4.
The septimal major third has a characteristic brassy sound which is much less sweet than a pure major third, but is classed as a 9-limit consonance. Together with the septimal minor third of 7/6, it makes up the septimal major triad, or supermajor triad. However, in terms of the overtone series, this is a utonal rather than otonal chord, being an inverted 6:7:9, ie a 1/6:1/7:1/9 chord.
In the early meantone era the interval made its appearance as the alternative major third in remote keys, under the name diminished fourth. Tunings of the meantone fifth in the neighborhood of Zarlino's 2/7 comma meantone will give four septimal thirds among the twelve major thirds of the tuning; this entails that three septimal major triads appear along with one chord containing a septimal major third with an ordinary minor third above it, making up a wolf fifth.
[edit] References
Hermann Helmholtz and Alexander Ellis (trans), On the Sensation of Tone, Dover Publications
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