The circle of fifths

Twelve keys arranged like a clock. Each step clockwise goes up a fifth and adds one sharp; counter-clockwise adds one flat.

Outer ring: major keys. Inner ring: their relative minors. G sits between C and D, its closest keys; Em is its relative minor.

Read it in three steps

Put your finger on a key. The key itself and its two neighbours are the I, IV and V chords; the inner ring under those three gives vi, ii and iii.

Key of G

Key of E: A (IV) and B (V) on either side, C♯m the relative minor.

Changing key a move to a neighbour sounds smooth; a jump across the circle sounds dramatic.
Key signatures count steps from C: G has 1 sharp, D has 2, F has 1 flat.
Easier keys guitar likes the sharp side (G D A E); horns like the flat side.

See every key with chord diagrams in chords in every key, find a song's key with the key finder, or change it with the chord transposer.