thumb|350x350px|Blue notes (in blue): 3, (4)/5, 7In jazz and blues, a blue note (or worried note) is a note that—for expressive purposes—is sung or played at a slightly different pitch from standard. Typically the alteration is between a quartertone and a semitone, but this varies depending on the musical context.
Origins and meaning
The blue notes are usually said to be the lowered third, lowered fifth, and lowered seventh scale degrees. The lowered fifth is also known as the raised fourth. 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". Bent or "blue notes", called in Ireland "long notes", play a vital part in Irish music.
Theory and measurement
Music theorists have long speculated that blue notes are intervals of just intonation not derived from European 12-tone equal temperament tuning. Just intonation musical intervals derive directly from the harmonic series. Humans naturally learn the harmonic series as infants. This is essential for many auditory activities such as understanding speech (see formant) and perceiving tonal music. In the harmonic series, overtones of a fundamental tonic tone occur as integer multiples of the tonic frequency. It is therefore convenient to express musical intervals in this system as integer ratios (e.g. = octave, = perfect fifth, etc.). The relationship between just and equal temperament tuning is conveniently expressed using the 12-tone equal temperament cents system. Just intonation is common in music of other cultures such as the 17-tone Arabic scale and the 22-tone Indian classical music scale. In African cultures, just intonation scales are the norm rather than the exception. 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. This note is commonly slurred with a major third justly tuned at (386 cents) refer to as a "neutral third". This bending or glide between the two tones is an essential characteristic of the blues. The barbershop quartet idiom also appears to have arisen from African American origins.