Blue note

[6] Music theorists have long speculated that blue notes are intervals of just intonation[7][8][9][10][11][12] not derived from European 12-tone equal temperament tuning.

It is therefore convenient to express musical intervals in this system as integer ratios (e.g. 2⁄1 = octave, 3⁄2 = perfect fifth, etc.).

[15] As the blues appears to have derived from a cappella field hollers of African slaves, it would be expected that its notes would be of just intonation origin closely related to the musical scales of western Africa.

[17] This note is commonly slurred with a major third justly tuned at 5⁄4 (386 cents)[17] in what Temperley et al.[18] refer to as a "neutral third".

[17] The eleventh harmonic (i.e. 11⁄8 or 551 cents) as put forward by Kubik[9] and Curry[10] is also possible as it is in the middle of the slur between the perfect fourth at 4⁄3 and 7⁄5.

[17] Kubik[9] and Curry[10] proposed 7⁄4 as it is commonly heard in the barbershop quartet harmonic seventh chord.

[17] It should not be surprising that blue notes are not represented accurately in the 12-tone equal temperament system, which is made up of a cycle of very slightly flattened perfect fifths (i.e. 3⁄2).

The just intonation blue note intervals identified above all involve prime numbers not equally divisible by 2 or 3.

The blues has likely evolved as a fusion of an African just intonation scale with European 12-tone musical instruments and harmony.