How to get a handle on scale theoryIf you play all of the white notes on a keyboard, in an upward direction starting with the note ‘C’, you will play C major scale. This is often why keyboard ...

The 9th (D) is nine notes above the original root when we play a two-octave C major scale. This, plus the inclusion of the b7 (Bb), is why this chord is called C9, instead of Cadd9.