blue note
A note played at a slightly lower pitch than standard, characteristic of blues and jazz.
In Depth
Blue notes are the emotional heart of the blues. They typically involve flattening the third, fifth, or seventh degrees of the major scale by a quarter tone or so, creating pitches that fall between the cracks of the piano keyboard. Blue notes reflect the microtonal vocal traditions of West African music filtered through the African American experience. They give blues, jazz, rock, and soul their characteristic bittersweet quality that cannot be accurately reproduced on fixed-pitch instruments.
Blue notes exist in the spaces between piano keys, which is why the best blues pianists crush two adjacent keys simultaneously to approximate the sound that guitarists and singers produce naturally.