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What is I–IV–V?
The three primary chords of a key — built on its first, fourth, and fifth notes. Almost every blues, folk tune, and early rock song lives inside these three chords.
Number the chords of a with Roman numerals and the ones built on the first, fourth, and fifth — I, IV and V — are the primary chords, the strongest and most closely related in the key. Together they can harmonise a huge share of Western melody, which is why 'three chords' is shorthand for a whole tradition of folk, country, blues, and early rock. The is made of nothing but I, IV and V.
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