It was a piano chord chart that got me started in piano playing when I was about 14 years old, and over the years I probably have purchased a couple dozen more chord charts of all kinds, from simple to complex. In this short video you’ll see a chart that shows the most-used chords in the key of C — known as the primary chords — as well as the secondary chords as well. I think you’ll find it facinating. Here are links to the two courses mentioned in the video: Chord Piano Crash Course