What music theory should I learn if I want to know what chords go together on the guitar?

You should learn about scales, the chords that can be played on different notes of the scale, alternate chords, the Circle of Fifths, and more.

Understanding scales is the foundation. Once you know a scale, you can find all the diatonic chords — the chords built naturally from that scale’s notes. These are the chords that belong together in a key, and knowing them makes choosing chord progressions straightforward rather than guesswork.

The Music Theory Companion Android app is a good resource for exploring these concepts. It shows you every scale, the diatonic triads and seventh chords for each scale, alternate chord options, and the Circle of Fifths — all on an interactive piano keyboard and guitar fretboard. That combination of visual and audio feedback makes abstract theory concrete.

Start with the major and natural minor scales, learn the diatonic chords for each, and work from there. Most of the chord progressions you hear in popular music come directly from these foundations.