9 chords include both the 7th (b7th or 7th) and 9th tones of the major scale. We'll also include the very similar 7b9 and 7#9 chords for comparison since they contain altered 9ths. Let's study the chord tones of the examples below.
9 chords contain the 1st, 3rd, 5th, b7th, and 9th notes of the major scale. As is common, the above shape omits the 5th.