Three chords can harmonise every note of a major scale
Wild Thing by The Troggs gets by on just three chords, and it is hardly unusual. In any major key, the chords built on the first, fourth and fifth notes of the scale together contain every note in it, which is why countless folk, blues and rock songs never need anything more.
A chord progression is simply a sequence of chords, and in much Western music it establishes or undermines the key of a piece. Theorists label chords with Roman numerals according to the scale note they are built on: uppercase for major, lowercase for minor. The three major chords of a major key are the tonic (I), subdominant (IV) and dominant (V). The same scale also yields three minor chords, ii, iii and vi, plus a diminished chord on the seventh note, the leading tone. A dominant seventh chord, V7, pulls especially strongly back toward I.
Numerals make songs portable. Rock and blues players think of the 12-bar blues as a pattern of I, IV and V chords, so a bandleader can call any key and the rhythm section will know what to play. That three-line blues form powered much of the work of Chuck Berry and Little Richard. Even simpler, some pieces just rock between two chords: many classical melodies alternate tonic and dominant, and the Isley Brothers' Shout sits on I and vi throughout. Funk tracks may ride one chord for an entire song, while bebop tunes can change chords once or twice in every bar of a 32-bar form.
Adding the sixth-degree chord gives I–vi–IV–V, nicknamed the doo-wop or 50s progression, heard in Blue Moon from 1934 and Heart and Soul from 1938. Today I–V–vi–IV is extremely common. Swapping IV for ii leads into the jazz staple ii–V–I, an example of circle progressions in which each root rises by a fourth, described as the strongest of all harmonic motions. Gershwin's I Got Rhythm gave jazz its famous rhythm changes.
Scales can also serve as bass lines. Seventeenth-century composers loved descending basses, and Pachelbel's canon harmonises the falling major scale in similar ways. In 1984 Steedman proposed recursive rewrite rules to generate valid jazz chord sequences.
Source: Chord progression
