Known as “the devil’s interval” since medieval times you can’t play jazz without it. The first accepted harmonic interval in Gregorian chant was the perfect 5th and the note a half step down created such dissonance that it was banned.
Jazz harmony has its distinctive sound through the use of 7th chords. Whenever people want to sound “jazzy” when they play the piano they play 7th chords, because it makes a basic triad sound more sophisticated. The 7th is the first extension to a basic 3- note chord.
The dominant 7th chord is the one with flat 5 interval, it happens between the 3rd and the 7th note of the chord. This interval is so important to the overall sound of the chord, many pianists will play just this intervals as an accompaniment in their left hand as they solo in their right hand.
Whenever you see chord symbols in a jazz lead sheet they usually have this type of chord in it somewhere:
This is not the only circumstance you come across the flat 5 interval in jazz, you also come across this interval in the blues scale. This is the main note that distinguishes a blues scale from a pentatonic scale. This note is the one I like to call the “melancholy note” because it brings out that tinge of longing within the scale. The African-Americans who were enslaved were longing for freedom and by adding that note to the pentatonic scale gives it a melancholy feeling. You do find this sound in Indian Classical music as well and it is used in precisely the same way as a moment of tension seeking release.
Because jazz is built on the blues and is always seeking to grow in its sophistication through harmonic extension and melodic embellishment, the tri-tone is ubiquitous in jazz.