Tritone substitution is a common harmonic technique used in music, particularly in jazz and other forms of popular music.
It involves replacing a dominant seventh chord with another dominant seventh chord that is a tritone (three whole steps) away.
For example, in the key of C major, the dominant seventh chord is G7. The tritone interval within the G7 chord is between the notes B and F. By replacing the G7 chord with a chord whose root is a tritone away, we get a Db7 chord.
This chord also contains the notes B and F, but in a different order, and it creates a similar harmonic tension and resolution as the G7 chord.
Tritone substitution is often used as a way to create harmonic interest and tension in a musical composition, and to create chromatic movement in the bassline or melody.
It can also be used as a way to modulate to a different key or tonality.
In jazz, tritone substitution is often used as a way to create more interesting and complex chord progressions, and to provide more harmonic variety and interest in improvised solos. It is a common and essential tool for jazz musicians and composers.