The function of mode is to determine the style of your music. The role of tonality is to determine the color and pitch layout of your entire music. The application of mode and key in music is very flexible. Generally speaking, in songs, modulation is normal. But there are very few modulations, and very few songs have both major and minor modes. Because this change in mode directly affects the change in music style, a song usually does not need such a big conflict and contrast. Mode changes are only made in relatively large-scale music that requires strong conflict and contrast. The accompaniment and melody must be unified in terms of mode and key. Generally speaking, when using the major key, we usually use the natural major key. When using minor keys, we usually use harmonic minor keys. Moreover, this does not mean that you cannot downgrade the natural major key by six steps from beginning to end. You can drop it by six steps at any time or restore it at any time. The same is true for minor keys, whether it appears in a melody or a chord.