The scale in C major is C D E F G A B, which stands for do, re, mi, fa, sol, la and xi. Among them, the first, fourth and fifth grades, that is, the chords represented by do fa sol, are the main chords, and the chords composed of major triad (C F G), second, third and sixth grades, that is, re Milla, are the minor chords, with a triad followed by an m(dm em am). The chord formed by the seventh sound xi is called the leading chord, which is a decreasing chord and is generally not used. So to sum up, C Dm Em F G Am and G7 (pentatonic seventh chord) are the basic chords in C key.

The same is true of other majors. Major triad of 1, 4,5, minor chords of 2,3,6, and 7 chord of 5 are the basic chords of this major.