In jazz and blues, a blue note (also "worried" note) is a note that—for expressive purposes—is sung or played at a slightly different pitch than standard. Typically the alteration is between a quartertone and a semitone, but this varies among performers and genres. Blue notes may be marked by a glissando or trill.
The blue notes are usually said to be the lowered third, lowered fifth, and lowered seventh scale degrees. The musicologist Peter van der Merwe also includes the lowered sixth, noting it as the least frequent type of blue note. The lowered fifth is also known as the raised fourth.
Blue notes are used in many blues songs, in jazz, and in conventional popular songs with a "blue" feeling, such as Harold Arlen's "Stormy Weather." Country blues, in particular, features wide variations from the diatonic pitches with emotive blue-notes. Blue notes are often seen as akin to relative pitches found in traditional African work songs. Blue notes are also prevalent in English folk music and Irish traditional music (in Ireland they are called long notes).
Though the blues scale has "an inherent minor tonality, it is commonly 'forced' over major-key chord changes, resulting in a distinctively dissonant conflict of tonalities". A similar conflict occurs between the notes of the minor scale and the minor blues scale, as heard in songs such as "Why Don't You Do Right?", "Happy" and "Sweet About Me".
In the case of the lowered third over the root (or the lowered seventh over the dominant), the resulting chord is a neutral mixed third chord.