How to read the circle of fifths
What each key's position on the circle means, and how to use it to read signatures and choose chords.
The idea behind the circle
The circle of fifths orders the 12 major keys by harmonic closeness, not alphabetically. Starting at C and moving clockwise, each key sits a perfect fifth away from the previous one (C, G, D, A...). Two neighboring keys on the circle share almost all their notes, which is why modulating between them sounds natural.
How to read key signatures from it
Moving one step clockwise adds one sharp to the key signature; moving counter-clockwise adds one flat. C major has no alterations, G major has one sharp, D major has two, and so on. It's a much faster way to recall key signatures than memorizing each one individually.
Relative minors
Every major key has a relative minor that shares the exact same key signature — it sits three semitones below the major tonic (or, on the circle, in the same segment, marked separately). A minor is the relative of C major, E minor of G major, and so on for each one.
A practical use: choosing the next chord
If you're writing and unsure where to take a progression, neighboring keys on the circle almost always work as a natural modulation, since they share most of their notes with the starting key.