Structure
Read clockwise from C, the sharps accumulate one at a time: G has one sharp, D has two, A has three. Read anti-clockwise, the flats accumulate the same way: F has one flat, B-flat has two, E-flat has three.
Inside the outer ring of major keys sits an inner ring of relative minors, each a minor third below its major partner. C major and A minor share a key signature; G major and E minor share theirs. The diagram is the master map of tonal music: every key signature, every chord progression, every modulation can be read off it at a glance.
Harmony
Most tonal progressions move anti-clockwise around the circle, which is why root motion by descending fifth is the deep grammar of Western harmony. The full ii to V to I cadence is three consecutive anti-clockwise steps. Bach's chorales, Mozart's sonatas, and Chopin's nocturnes all draw on this engine; jazz extends it by stringing fifths together into longer chains.
At the keyboard
At the keyboard, every pianist should be able to play scales and arpeggios around the circle from memory, both clockwise and anti-clockwise. The exam-board scales requirements — at every grade from Initial through Grade 8 — are organised around it. Practising scales in this order also reveals the subtle hand-position shifts each new key requires, building physical familiarity alongside theoretical knowledge.
When sight-reading, glance at the key signature and place yourself on the circle before the first note. Knowing whether you are in A major (three sharps, three steps clockwise from C) or in E-flat major (three flats, three steps anti-clockwise) tells you instantly which black keys are in play and which chord centres to expect. Pair the wheel with a metronome session and the abstract diagram becomes a physical reflex.