Jazz Harmony
Every chord gets a seventh, most get more, and the ii V I is the sentence everything is built from. The reason substitutions work is simpler than it looks: the third and the seventh stay where they are, and everything else is negotiable.
- sevenths minimum
- ii V I
- third and seventh stay
In which styles
- Jazz
- Bossa nova
- Soul
- Musical theatre
- Film music
In short
Jazz harmony starts from a plain observation: a seventh chord has one interval inside it, between the third and the seventh, that defines what the chord does. Keep those two notes and you can change the root, add extensions, alter the fifth, and the progression still functions. That is why a tritone substitution works: the substitute chord contains the same third and seventh, with the roles swapped.
Rich, and slightly unresolved even at rest. A jazz chord holds notes that classical harmony would have made move on. Living with that permanent low-level tension is most of what makes the style sound like itself.
Where it sits in the phrase
The ii V I is a complete sentence: preparation, tension, arrival. Most standards are chains of them, in different keys, stitched together. Learn to hear where one ends and the next begins and a thirty-two bar form stops being a wall of chord symbols.
Where you run into this
- Any standard, where the same three-chord sentence keeps returning in new keys
- A turnaround in the last two bars, sending you back to the top
- A blues, where the ii V shows up in bars nine and ten
The types side by side
Play the plain ii V I, then the same cadence with the tritone substitution, then with a rootless voicing, then with altered extensions on the V. Four versions, one function. What changes is colour and bass movement; what does not change is the pair of notes doing the work. Once you hear that, substitution stops being a rule to memorise.