Responses (1)
lets say your major chord is Cmaj7 ( c,e,g,b ) as the key chord (the piece is written in C major), then Cmaj7 is your root chord, or key. I like to assign a roman number to it. In this case the 'I'. Adding the ninth doesn't change that (c , e , g , b, d).
The most common progression in jazz (and in many other music styles) is II-V-I . Almost any song ends like that.
II means D minor7 (d , f , a, c) and V means G7 (g , b, d, f). Check that the three chords are built with notes from the C major scale.
It's always II minor and V is always dominant 7 as long as the chords are built with the notes of the corresponding key.