Theory
In Western classical music during the common practice period, chord progressions are used to structure a musical composition. The destination of a chord progression is known as a cadence, or two chords that signify the end or prolongation of a musical phrase. The most conclusive and resolving cadences return to the tonic or I chord; following the circle of fifths, the most suitable chord to precede the I chord is a V chord. This particular cadence, V-I, is known as an authentic cadence. However, since a I-V-I progression is repetitive and skips most of the circle of fifths, it is common practice to precede the dominant chord with a suitable predominant chord, such as a IV chord or a ii chord (in major), in order to maintain interest. In this case, the 50s progression uses a IV chord, resulting in the ubiquitous I-IV-V-I progression. The vi chord before the IV chord in this progression (creating I-vi-IV-V-I) is used as a means to prolong the tonic chord, as the vi or submediant chord is commonly used as a substitute for the tonic chord, and to ease the voice leading of the bass line: in a I-vi-IV-V-I progression (without any chordal inversions) the bass voice descends in major or minor thirds from the I chord to the vi chord to the IV chord.
Read more about this topic: 50s Progression
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