Prove it — just do the practice exercise.

CMaj9 Root Position

C: R
E: 3
G: 5
B: 7
D: 9

The Maj9 extends Maj7 with the 9th (= 2nd scale degree):

• R – 3 – 5 – 7 – 9

The 9th adds color without changing stability. Lush and modern.

Example: CMaj9 = C + E + G + B + D