Circle of Fifths

So much in music theory depends on the concept of the circle of fifths. It’s one of the easiest ways to learn the keys and relationship between keys. Take a look at a piano, starting on the lowest C possible. If you go clockwise (up) by perfect 5ths, or counter-clockwise (down) by perfect 4ths you will cycle through all the keys in 12 steps. Here’s a familiar diagram of the famous circle of 5ths:

Notice the 3 cases with enharmonic spelling: (C#/Db, F#/Gb, B/Cb). These are interchangeable and enharmonically equivalent.
The circle of fifths is sometimes called the circle of fourths because if you travel counter-clockwise, the steps you will be taking to arrive at the next chord will be Perfect 4ths.

Basically, you can cycle through all the chords in a progression with this circle. You can go in a stepwise motion from one chord to the next in either direction and it will prove as a useful tool in studying and/or using harmonic progression. When used in combination with the relative minor, it expands the possibilities even more.