Harmony is actually the art form of the relationship between these chords. The harmony has two major functions. One is to support a melody to provide the meaning for the melody, for the tune. The other function, major function, is to push the music forward. A good harmony is like the waves in a river. Keeps pushing the music forward until the river, the water reaches the ocean. Then the next thing is modulation. Modulation is simply meaning that you go out of your home key. So if you have A major or so, and all of a sudden you decided to spend some time in D major. So you are out of your home key, you go to another key, that's what we call modulation. So why do we need modulation? Why is that important? Well, if you have a short composition like this one, over the rainbow. Then you are happy with just one little song. But if you deal with the symphony, that long and four movements. If you stay with one A major chord, that means you keep repeating these chords, and the music becomes static, and to put it simply boring. So we need to modulate. So it's like the king. We talked about a king. No matter how beautiful the palace is, the garden is, sometimes he would like to get out, and that's what modulation is. This borrowing or visiting is called a modulation. We just go out from your original key, your home key, and you go out. So in our example of A major here and a minor, any of these natural triads could become atonic. If you look at this example one. As long as they are major or minor triad, you can use that as atonic. By building, for example if you do D major, and then you have this. You have these chords in D major. Then its own relationship in a way everybody has its own cabinet of servants, chief of the staff. Although they all belong to the A major. In our case, the A major, the atonic key, and then reports to the king. So we usually only visit these chords, these territories. These are the territories the king can visit. Well, in the classical music, that's the case. Most comfortably, you go into these natural keys, natural keys. So in the A major case, we have A major. Sometimes we go b minor, c# minor, D major, E major, f# minor. We don't go to, this is a diminished chord, so it has a kind of a sad instrument feeling. Itself it cannot be atonic. So this is the classical period. But sometimes just goes close range doesn't make the music fresh enough. Sometimes we want to go further away just to be more exotic. So one of the common practices during the classical period is to step out from the A major, and where can you go? Of course in theory, you can go anywhere you want. One of the common practice during the classical period is to borrow the key. Go to these keys, these triads. That based on these tries that belong to the parallel minor, instead of the A major, you go to a minor. You could also go to C major, which doesn't belong to the A major but belongs to its parallel brothers. The king's brother. The king feels he can go anywhere he wants to go, because he's the king and he still owns everything. So the king can borrow from his brother. But as you said, this is a monarchy not a democratic country. So this means that there's a strict hierarchy. So let me tell you what hierarchy in music is. Hierarchy means some notes are more important than the other. Just like in society, hierarchy in society. Some people are more important, has more power than the other. But none of it is more important than the atonic, the king. So everybody bows to the king. So but in music, we only have 12 notes on the piano. So we have this chromatic scale. Then we'll repeat in different octaves. So this group of 12 notes. In order to manage a large symphony, we must establish origin to push the music forward. Have a very clear musical direction. So this is what we when hierarchy comes handy. Yes. Without that I guess the story would get lost. The fact that Beethoven lived in a monarchic society when he wrote symphony number seven as quite well suited to having this metaphor of the king.