The study of chords can go on forever from here. For example, you can keep adding notes by using a “one, three, five, seven, nine” pattern (“nine” is “two” up an octave). You can alter notes in any chord (try Cmaj9#5, it’s C, E, G#, B, D). There are many more ways of using chords from outside the scale, and many ways of building chord progressions without ever thinking about one particular scale. If you are interested in this study I would recommend a jazz theory book or a book on jazz piano. You’ll find some at most music stores.
It was difficult to decide where to end part four of this book. I wanted to cover the basic concepts that will help you understand the harmonies you hear, but there is no clear line between the basics and more “advanced” genre-specific concepts. I’m tempted to try to prepare you for many different genres. I could give examples of common chord progressions, I could talk about the functions of chords, and I could show you my harmonic analysis of many different pieces of music. But this would make this book very long, and these things may not be of interest to many of you. However, even if you understand this whole chapter and have done the exercises, you may encounter situations where you don’t understand the harmony, or you can’t figure out a harmony by ear. My advice is to learn these songs (you may have to ask a teacher or search online to figure out the harmony) and analyze the chords. If there was one chord you couldn’t figure out, notice how that chord relates to the key of the song, and to the other chords in the song. If it goes outside the scale, how exactly does it do that? I hope that this chapter has given you the tools to do this, so you can learn about chord progressions and harmonies you like and be able to use them in your musical creations.