The theory of guitar-chords respects
harmonic conventions of Western music. Discussions of basic guitar-chords rely on
fundamental concepts in
music theory: the twelve notes of the octave, musical intervals, chords, and chord progressions.
Intervals {{listen The octave consists of twelve notes. Its natural notes constitute the
C major scale, (
C,
D,
E,
F,
G,
A,
B, and C). The intervals between the notes of a
chromatic scale are listed in a table, in which only the
emboldened intervals are discussed in this article's section on fundamental chords; those intervals and other seventh-intervals are discussed in the section on intermediate chords. The
unison and octave intervals have perfect
consonance. Octave intervals were popularized by the jazz playing of
Wes Montgomery. The perfect-fifth interval is highly consonant, which means that the successive playing of the two notes from the perfect fifth sounds harmonious. A
semitone is the distance between two adjacent notes on the
chromatic circle, which displays the twelve notes of an
octave. As indicated by their having been emboldened in the table, a handful of intervals—thirds (minor and major), perfect fifths, and minor sevenths—are used in the following discussion of fundamental guitar-chords. As already stated, the perfect-fifths (P5) interval is the most harmonious, after the unison and octave intervals. An explanation of human perception of harmony relates the
mechanics of a
vibrating string to the
musical acoustics of
sound waves using the
harmonic analysis of
Fourier series. When a string is struck with a finger or pick (plectrum), it vibrates according to its
harmonic series. When an open-note C-string is struck, its harmonic series begins with the terms (C,C,G,C,E,G,B,C). The root note is associated with a sequence of intervals, beginning with the unison interval (C,C), the octave interval (C,C), the perfect fifth (C,G), the perfect fourth (G,C), and the major third (C,E). In particular, this sequence of intervals contains the thirds of the C-major chord {(C,E),(E,G)}.
Perfect fifths The perfect-fifth interval is featured in guitar playing and in sequences of chords. The sequence of
fifth intervals built on the C-major scale is used in the construction of triads, which is discussed below.{{efn|1=This sequence of
fifths features the
diminished fifth (b,f), which replaces the perfect fifth (b,f) containing the
chromatic note f, which is not a member of the
C-major key. The note f (of the C-major scale) is replaced by the note f in the
Lydian chromatic scale. {c d e fis g a b c' }}} { }
Cycle of fifths Concatenating the perfect fifths ((F,C), (C,G), (G,D), (D,A), (A,E), (E,B),...) yields the
sequence of fifths (F,C,G,D,A,E,B,...); this
sequence of fifths displays all the notes of the octave. This sequence of fifths shall be used in the discussions of chord progressions, below.
Power chord The perfect-fifth interval is called a power chord by guitarists, who play them especially in blues and
rock music.
The Who's guitarist,
Peter Townshend, performed power chords with a theatrical windmill-strum. The
major triad has a root, a
major third, and a fifth. (The
major chord's
major-third interval is replaced by a
minor-third interval in the
minor chord, which shall be discussed in the next subsection.) For example, a
C-major triad consists of the (root, third, fifth)-notes (C, E, G). The three notes of a major triad have been introduced as an
ordered triplet, namely (root, third, fifth), where the major third is four semitones above the root and where the perfect fifth is seven semitones above the root. This type of triad is in closed position. Triads are quite commonly played in open position: For example, the C-major triad is often played with the third (E) and fifth (G) an octave higher, respectively sixteen and nineteen semitones above the root. Another variation of the major triad changes the order of the notes: For example, the C-major triad is often played as (C,G,E), where (C,G) is a perfect fifth and E is raised an octave above the perfect third (C,E). (C,E,G), for example. This reduces the Diophantine-approximation error of the major third in equal-temperaments, compared to just intonation. --> Alternative orderings of the notes in a triad are discussed below (in the discussions of
chord inversions and
drop-2 chords). In popular music, a subset of triads is emphasized—those with notes from the three
major-keys (C, G, D), which also contain the notes of their
relative minor keys (Am, Em, Bm).
Progressions traditionally enumerated with the Roman numerals I, ii, iii, IV, V, vi, vii. Its major-key sub-progression C–F–G (I–IV–V) is conventional in popular music. In this progression, the minor triads ii–iii–vi appear in the relative minor key (Am)'s corresponding chord progression. The major chords are highlighted by the
three-chord theory of
chord progressions, which describes the
three-chord song that is archetypal in popular music. When played sequentially (in any order), the chords from a three-chord progression sound harmonious ("good together"). The most basic three-chord progressions of Western harmony have only major chords. In each key, three chords are designated with the
Roman numerals (of musical notation): The tonic (I), the subdominant (IV), and the dominant (V). While the chords of each three-chord progression are numbered (I, IV, and V), they appear in other orders. In the 1950s the I–IV–V chord progression was used in "
Hound Dog" (
Elvis Presley) and in "
Chantilly Lace" (
The Big Bopper). Major-chord progressions are constructed in the harmonization of
major scales in triads. For example, stacking the
C-major scale with thirds creates a chord progression, which is traditionally enumerated with the Roman numerals I, ii, iii, IV, V, vi, vii; its sub-progression C–F–G (I–IV–V) is used in popular music, as already discussed. Further chords are constructed by stacking additional thirds. Stacking the dominant major-triad with a minor third creates the dominant seventh chord, which shall be discussed after minor chords.
Minor A minor chord has the root and the fifth of the corresponding major chord, but its first interval is a minor third rather than a major third: : Minor chords arise in the harmonization of the major scale in thirds, which was already discussed: The minor chords have the degree positions ii, iii, and vi. : Minor chords arise as the tonic notes of
minor keys that share the same
key signature with major keys. From the major key's I–ii–iii–IV–V–vi–vii progression, the "secondary" (minor) triads ii–iii–vi appear in the relative minor key's corresponding chord progression as i–iv–v (or i–iv–V or i–iv–V7): For example, from C's vi–ii–iii progression A–D–E, the chord E is often played as E or E7 in a minor chord progression. Among basic chords, the minor chords (D,E,A) are the tonic chords of the relative minors of the three major keys (F,G,C): : The technique of changing among relative keys (pairs of relative majors and relative minors) is a form of
modulation. Minor chords are constructed by the harmonization of
minor scales in triads.
Seventh chords: major–minor chords with dominant function Adding a
minor seventh to a major triad creates a
dominant seventh (denoted V7). In music theory, the "dominant seventh" described here is called a major-minor seventh, emphasizing the chord's construction rather than its usual function. Dominant sevenths are often the dominant chords in three-chord progressions, These progressions with seventh chords arise in the harmonization of major scales in seventh chords.
Twelve-bar blues Be they in major key or minor key, such I–IV–V chord progressions are extended over
twelve bars in popular music—especially in
jazz,
blues, and
rock music. The twelve-bar blues structure is used by McCartney's "3 Legs", which was noted earlier. ==Playing chords: open strings, inversion, and note doubling==