Extensions

jazz harmony uses many color tones (notes other than root, third, fifth, or seventh)
we generally strive for a consistent texture - no "colorless" chords!
the most common are the ninth and thirteenth
unless explicitly altered with a b or #, these come from the major scale of the root
thus the unaltered ninth is always a whole step above the root
the unaltered sixth is a whole step above the fifth, except for diminished chords
in a chorale voicing, these notes may be added above or below alto the alto voice
depending on the melody, you may find you can use the 9 or 6 or both in your voicing
we generally avoid the 6 on ii chords (including secondary ii chords)
for diminished and half-diminished seventh chords, choose any one or two notes a whole step above chord tones

Jazz Piano Holiday