Every key is a step of a fifth from its neighbour, and twelve fifths later you're home — the whole family of keys on one wheel, closest relatives side by side. Tap a key to hear its chord, flip to its shadow minor, and walk the four chords every pop song is built from.
The circle of fifths: each step clockwise is a perfect fifth (×3/2 in pitch, +7 semitones), and it takes exactly twelve to return — because 12 fifths ≈ 7 octaves. Tap a key and the page plays its triad live (root + major third + fifth) through Web Audio; the key signature (sharps clockwise, flats counter-clockwise) and the relative minor (a minor third below) are read straight off the wheel. The I–V–vi–IV button walks the four chords under a huge share of pop songs — real, playable harmony, no recordings.
The wheel is 12-tone equal temperament (the fifth is the tempered 700 cents, not the pure 701.955), and 'relative minor' / key-signature counts are standard theory conventions; enharmonic spellings (F♯ vs G♭) are collapsed. The pitches, intervals and the twelve-fifths-close-the-circle fact are exact and generated live.