Jazz
The most important harmonic rule in jazz is the simplest: the triad is not the basic chord, the seventh chord is. Where a pop song plays C, F and G, a jazz band plays Cmaj7, F7 and G7, and everything else follows from that.
- sevenths minimum
- swing
- ii V I
In which styles
- Jazz
- Bossa nova
- Fusion
- Musical theatre
In short
On top of the sevenths come ninths, elevenths and thirteenths, and altered versions of all of them. The result is a harmony where no chord is fully at rest, so the music can keep moving without ever being forced to stop.
Conversational. Jazz sounds like several people deciding things in real time, because that is what is happening.
How the style works
Swing means two things at once and they are regularly conflated. It is a subdivision, where two eighths are played long-short. It is also a feel, produced by placement and by the ride pattern, and the subdivision on its own does not produce it.
Where you run into this
- A standard where the same three-chord sentence keeps returning in new keys
- A soloist whose line makes the chord changes audible without the band
- A ride cymbal carrying the whole time feel
What you leave out
Bare triads, which sound empty in this context because there is no direction in them. And scales run for their own sake, which is the most common way a technically correct solo says nothing.