C key: The main note is C, which is called C key. G key: The main note is G, which is called G key. D key: The main note is D, which is called the key of D. (And so on) The tonic is the starting note, the first note. CDEFGAB represents the seven notes of 1234567 respectively, and the key of C with C as the main note is 1234567, and the key of G with G as the main note is 2345671. This call is used in songs to modulate the pitch according to different people's pitch problems. When used in piano music or other instrumental music, it reflects the theme image of the music according to the tonality.