Many people have asked "John, what is a tritone substitution and how will it improve my life as a jazz musician?" Tritone (meaning "three tones" or three whole steps) substitution is a simple way of enhancing a basic chord progression that involves the dominant seventh chord. We all know the ii-V-I progression is the most fundamental harmonic building block of a standard jazz tune, i.e., Dm7-G7-CMaj7 in the key of C major or Dm7b5-G7-Cm7 in C minor. The tritone sub simply involves replacing the dominant seventh chord (the G7 in this case) with another dominant seventh chord with a root that has an intervallic relationship with the original of a tritone or three whole steps. For example Dm7-G7-CMaj7 becomes Dm7-Db7-CMaj7. If we examine the chord tones of the tritone sub we find that the all important leading-tone (B) and seventh (f) of the original G7 are still there but with the addition of two new tension tones (Db and Ab). These additional tones can be considered as the b5 and b9 respectively of the original G7 creating the "altered dominant" harmony which is so important to an authentic jazz sound. Try out this concept in any tune with dominant seventh chords that resolve down by fifth, the traditional and most common resolution. If the dominant seventh resolves down by a minor second or half-step it is most likely already a tri-tone sub that was written into the tune. In this case try using the original dominant as an alternate.