Circle of Fifths
Every key, one map. Neighbors on the circle share the most notes - they're the easiest keys to modulate to.
What is the circle of fifths?
It's the 12 major keys arranged so each step clockwise moves up a perfect fifth: C, G, D, A, E, B, F#, C#, G#, D#, A#, F, back to C. Each step adds one sharp (or removes one flat), so the keys next to each other differ by a single note.
How producers use it
Pick your key, then borrow chords from its neighbors for a lift that still sounds in-key. The key one step clockwise is your dominant (V), one step counter-clockwise is your subdominant (IV) - the two most common places a progression wants to go.
Relative minors
The inner ring holds each major key's relative minor, three semitones below the major root: C major shares its notes and its diatonic chords with A minor. Same palette, different tonic - that's why swapping between them keeps every chord usable.