The blues lives in notes that fall between the piano keys.
A blue note is sung or played slightly flatter than the standard pitch, often somewhere a piano simply can't reach. Measurements of early blues recordings suggest these notes cluster near simple whole-number frequency ratios, hinting at roots in West African tuning rather than European keyboards.
In jazz and blues, a blue note, sometimes called a worried note, is pitched a little differently from the standard note for expressive effect, typically lower by somewhere between a quarter-tone and a semitone. The notes most often treated this way are the third, seventh and fifth of the scale. On a piano, which can only offer the fixed semitones of equal temperament, players approximate them by lowering a note a full semitone or by sliding between two neighbouring notes. Singers, guitarists and horn players can bend and slide into the gaps.
That bending matters. A blue third is commonly slurred up toward the major third, creating what researchers call a neutral third, neither major nor minor. The blues scale also has a minor flavour that is regularly played over major-key chords, and the resulting clash is part of its bittersweet sound.
Where do these pitches come from? Theorists have long speculated that blue notes belong to just intonation, tuning based directly on the simple ratios of the natural harmonic series, rather than to the European 12-note system. A cluster analysis of many blue notes from early blues recordings found the blue third centring near a ratio of 6:5, a pure minor third. The blue seventh appeared at two common positions, near 7:4 and 9:5, sometimes within the same song, and 9:5 was surprisingly the more common.
Because the blues appears to have grown from the unaccompanied field hollers of enslaved Africans, many scholars expect its tuning to echo West African scales, where just intonation is the norm. On this reading, the blues is a fusion of an African way of tuning with European instruments and harmony. Similar bent notes also appear in English folk music and in Irish music, where they are called long notes. Either way, the blues gave rise to jazz, and its 'wrong' notes became some of the most expressive in popular music.
Source: Wikipedia — Blue note · Text summarised from Wikipedia (CC BY-SA 4.0)
