There are 12 major keys one can play in (more if you count enharmonic keys, but we won’t here). To learn those 12 keys, it is useful to learn the “Circle of Keys”. It is often called the “Circle of 5th” or the “Circle of 4ths”. It is all the same thing — just depends which way you move on the circle. Watch this 5-minute video on the circle:

Maybe you’ve heard professional musicians talk about the circle of 5ths as though it was some kind of inside information available only to pros. Not so — it is pretty easy to learn because of a couple “beads” on both sides of the circle. What it does is show you related chords. Look at the top of the circle — you’ll see “C”. Then look left and look right and you’ll see “G” and “F”.  In the key of C, those 3 chords are the most used of any chords — in fact they are called the “primary chords” for that key. Now look at any other point on the circle and then left and right and you’ll immediately know what are the most used chords in that key. Pretty easy, no? But also, as you progress around the circle clockwise you will see that each new key is 4 scale degrees above the last key. If you go counter-clockwise, you’ll see that each new key is 5 scales degrees above the previous key. That’s why it is usually called the “circle of 5ths”, but sometimes called the “circle of 4ths”. I suspect the most accurate name is the “circle of keys.” It’s very easy to memorize, because when moving clockwise after C and F are the letters Bb, Eb, Ab, and Db — which of course spells the word “bead.”  Then at the bottom of the circle is an F# or Gb (they are enharmonic tones — same sound, different name) and then the word “bead” again, followed by “G” and then you’re back to C. I’ll make a video about it soon so you can see how it works on the keyboard, but meanwhile, memorize that circle (if you don’t already have it memorized, that is.) You’ll find that memorizing that circle helps you learn music theory faster, key signatures better,  chord progressions more logical, major scales and minor scales make more sense, and even pentatonic scales more understandable.