vi–ii–V–I
In music, the vi–ii–V–I progression is a chord progression (also called the circle progression for the circle of fifths, along which it travels). A vi–ii–V–I progression in C major (with inverted chords) is shown below.[1]
It is "undoubtedly the most common and the strongest of all harmonic progressions" and consists of "adjacent roots in ascending fourth or descending fifth relationship", with movement by ascending perfect fourth being equivalent to movement by descending perfect fifth due to inversion.[2] For instance, in C major, the chords are Am–Dm–G–C, which have roots that descend by perfect fifth (or ascend by fourth), as shown below.[3]