Chord progressions are the basis of musical harmony, providing different relationships between the different notes that are played. As chords are based on scales, a guitar player can utilize different chord progressions to provide a backing to a preexisting melody, depending on its tempo and key signature. There are different types of guitar chord progressions that a musician can use, with varying numbers of chords. Some of the more common progressions include the three-chord I-IV-V and the twelve-bar blues progression.
Guitar chord progressions are written in Roman numerals. I, the root note, is based on the key of the song. II is the second interval, III third and so on. A three-chord progression with a song in the key of C using a C-F-G-G sequence would be written as I-IV-V-V. Minor intervals are denoted by writing the Roman numeral lowercase. I-IV-vi-V, the standard four-chord progression, represents C-F-A minor-G.
Simple guitar chord progressions tend to feature two alternating chords of the same scale. For example, a song in the key of C might have alternating C and G chords, or I-V-I-V. The order in which the chords are played is irrelevant; simple progressions utilize two chords throughout the piece. These alternating chords work for simple songs due to the use of fifth intervals, which mesh well harmonically.
The three-chord progression I-IV-V is the most commonly used type of guitar chord progression; it forms the basis of most rock music. For example, a song in the key of C has the chords C, F and G. The advantage of a three-chord progression is that between the three chords, all notes in the scale are present: a C chord contains C, E and G; an F chord contains F, A and the octave of C and a G chord contains G, B and D. Any note can form the root of the song, which allows more versatility in composing.
Another of the most-used guitar chord progressions is the twelve-bar blues progression. It is based on the three-chord progression, but the movement is prolonged over twelve measures. Popular in the 1950s, the chord progression is I-I-I-I-IV-IV-I-I-V-V7-I-I. V7 means that a seventh interval is added to the chord in addition to the standard triad/octave intervals; a V7 chord based on C would be G, B, D and F sharp.