Supplement: this requires a certain knowledge of music theory, interval and harmony to pass. Actually, the C9 mentioned above is a ninth chord. If we follow the calculation method in the normal music theory textbook, we should add a note (bB) with a seven-level drop, which consists of C, E, G, bB and D, and the bB is omitted here. According to the above relationship, the other six basic levels can be regarded as root sounds respectively, and their nine chords can be deduced.