What is subordinate chord and what is its use?

Simply put, it is "a dominant chord in a certain mode".

First of all, it needs to be clear that the function of polyphony is generally to enrich the harmony color, which is reflected in the mode of "out of tune", that is, temporarily moving from the original key to another key, but then returning to the original key.

Re-membership of chords helps to get out of tune.

So how do you value the genus?

Firstly, it is clear that each natural major and harmonic minor has only one dominant chord (major and minor). According to this property, taking C major as an example, we can know that its dominant chord is G7, and the dominant chord is a dominant chord composed of G as a temporary tonic and then upward, that is, D7 chord, which is the dominant chord in C major.

In the words of graph, it is C-G7 (genus of C key)-D7 (genus of G key, genus of C key).

There is another simple way. The second-order seventh chord in the original song is a subordinate chord.

Let's take c major as an example. The size of the second level is seven, that is, D7 is a heavy genus.