Chord progressions are the foundation of musical harmony, establishing various relationships between the various notes played. Because chords are based on scales, a guitarist can use a variety of chord progressions to provide a backing to a melody, depending on its tempo and key signature. A musician can use a variety of guitar chord progressions, each with a different number of chords. The three-chord I-IV-V and the twelve-bar blues progression are two of the most common progressions.
Roman numerals are used to write chord progressions on the guitar. The root note, I, is based on the song’s key. The second interval is II, the third is III, and so on. I-IV-V-V is a three-chord progression for a song in the key of C using the C-F-G-G sequence. Minor intervals are represented lowercase Roman numerals. C-F-A minor-G is represented the standard four-chord progression I-IV-vi-V.
Two alternating chords of the same scale are common in simple guitar chord progressions. A song in the key of C, for example, might have alternating C and G chords, or I-V-I-V. It makes no difference in which order the chords are played; simple progressions use two chords throughout the piece. Because of the use of fifth intervals, which mesh well harmonically, these alternating chords work well for simple songs.
The chord progression is made up of three chords. The most common type of guitar chord progression is I-IV-V, which is the foundation of most rock music. The chords C, F, and G, for example, are used in a song in the key of C. A three-chord progression has the advantage of containing all notes in the scale between the three chords: a C chord contains C, E, and G; a F chord contains F, A, and the octave of C; and a G chord contains G, B, and D. Any note can be used as the song’s root, allowing for greater compositional flexibility.
The twelve-bar blues progression is another popular guitar chord progression. The movement is based on a three-chord progression, but it is stretched out over twelve measures. The chord progression is I-I-I-I-IV-IV-IV-IV-I-I-V-V7-I-I, which was popular in the 1950s. V7 chords include a seventh interval in addition to the standard triad/octave intervals; for example, a V7 chord based on C would be G, B, D, and F sharp.