A few foundational chord progressions recur throughout Western music. The I–IV–V–I progression cycles through tonic, subdominant, dominant, and back to tonic and is the bedrock of countless songs across genres. In C major, this is C–F–G–C. The ii–V–I progression is the most important progression in jazz harmony; the ii chord provides smooth voice leading into the dominant V7, which then resolves to the tonic. In C major, this is Dm7–G7–Cmaj7.
The 12-bar blues form follows a three-line chord pattern, typically I–I–I–I / IV–IV–I–I / V–IV–I–I (with the last bar sometimes using V as a turnaround), and is the foundation of blues, rock and roll, and early jazz. In the key of A, this uses A7, D7, and E7. The I–V–vi–IV progression is one of the most common pop progressions; sometimes called the four-chord song or the Axis of Awesome progression, it has appeared in hundreds of hit songs. In C major, this is C–G–Am–F. To make transposition easier, the Nashville Number System represents chords as scale-degree numbers rather than letter names, so the same numeral pattern describes the progression in any key.
Beyond these core progressions, several harmonic devices add variety and motion. A deceptive cadence occurs when the dominant V chord resolves not to the expected tonic but to another chord, most commonly vi, creating surprise while still providing a sense of harmonic arrival. A secondary dominant is a dominant seventh chord that tonicizes a chord other than the tonic; in C major, for example, D7 functions as V7/V because it is the dominant of G. Secondary dominants introduce chromatic color and forward momentum without leaving the home key, and they are commonly used to add interest within otherwise diatonic progressions.