In music theory

Music theory

Music theory is the study of how music works. It examines the language and notation of music. It seeks to identify patterns and structures in composers' techniques across or within genres, styles, or historical periods...

, chord substitution is the use of a chord

Chord (music)

A chord in music is any harmonic set of two–three or more notes that is heard as if sounding simultaneously. These need not actually be played together: arpeggios and broken chords may for many practical and theoretical purposes be understood as chords...

 in the place of another related chord in a chord progression

Chord progression

A chord progression is a series of musical chords, or chord changes that "aims for a definite goal" of establishing a tonality founded on a key, root or tonic chord. In other words, the succession of root relationships...

. Jazz

Jazz

Jazz is a musical style that originated at the beginning of the 20th century in African American communities in the Southern United States. It was born out of a mix of African and European music traditions. From its early development until the present, jazz has incorporated music from 19th and 20th...

 musician

Musician

A musician is an artist who plays a musical instrument. It may or may not be the person's profession. Musicians can be classified by their roles in performing music and writing music.Also....* A person who makes music a profession....

s often substitute chords in the original progression to create variety and add interest to a piece. The substitute chord must have some harmonic quality and degree of function

Diatonic function

In tonal music theory, a diatonic function is the specific, recognized role of each of the 7 notes and their chords in relation to the diatonic key...

 in common with the original chord, and often only differs by one or two notes. Scott DeVeaux describes a "penchant in modern jazz for harmonic substitution."

Types

The ii-V substitution is when a chord or each chord in a progression is preceded by its supertonic

Supertonic

In music or music theory, the supertonic is the second degree or note of a diatonic scale, one step above the tonic. In music theory, the supertonic chord is symbolized by the Roman numeral ii in a major scale, indicating that the chord is a minor chord , or ii in a natural minor scale, indicating...

 (ii7) and dominant (V7), or simply its dominant. For example, a C major chord would be preceded by Dm7 and G7. Since secondary dominant

Secondary dominant

Secondary dominant is an analytical label for a specific harmonic device, prevalent in the tonal idiom of Western music beginning in the common practice period...

 chords are often inserted between the chords of a progression rather than replacing one, this may be considered as 'addition' rather than 'substitution'.
Chord quality alteration is when the quality of a chord is changed, and the new chord of similar root and construction, but with one pitch different, is substituted for the original chord, for example the minor sixth for the major seventh, or the major seventh for the minor.
The diminished seventh chord

Diminished seventh chord

A diminished seventh chord is a four note chord that comprises a diminished triad plus the interval of a diminished seventh above the root. Thus it is , or enharmonically , of any major scale; for example, C diminished-seventh would be , or enharmonically...

is often used in place of a dominant 7th chord. In the key of A Major the V chord, E dominant 7th (which is made up the notes E, G, B, and D) can be replaced with a G diminished seventh chord (G, B, D, F). If the diminished seventh chord (G) is followed by the I chord (A), this creates chromatic

Diatonic and chromatic

Diatonic and chromatic are terms in music theory that are most often used to characterize scales, and are also applied to intervals, chords, notes, musical styles, and kinds of harmony...

 (stepwise

Steps and skips

In music, a step, or conjunct motion, is a linear or successive interval between two pitches which are consecutive scale degrees. Any larger interval is called a skip , or disjunct motion...

 semitonal) root

Root (chord)

In music theory, the root of a chord is the note or pitch upon which a triadic chord is built. For example, the root of the major triad C-E-G is C....

 movement, which can add musical interest in a song mainly constructed around the interval of the fourth or fifth. The diminished seventh chord

Diminished seventh chord

A diminished seventh chord is a four note chord that comprises a diminished triad plus the interval of a diminished seventh above the root. Thus it is , or enharmonically , of any major scale; for example, C diminished-seventh would be , or enharmonically...

 on the sharpened second

Supertonic

In music or music theory, the supertonic is the second degree or note of a diatonic scale, one step above the tonic. In music theory, the supertonic chord is symbolized by the Roman numeral ii in a major scale, indicating that the chord is a minor chord , or ii in a natural minor scale, indicating...

 scale degree, II⁷, may be used as a substitute dominant, for example in C: II⁷ = D-F-A-C = B-D-F-A = VII⁷.

In a tritone substitution

Tritone substitution

In classical music, a substitute dominant is "a chord sufficiently akin to the dominant to be reasonably set against the tonic, and yet remote enough to give a chromatically expressive, large-scale dissonance to the structure"...

, the substitute chord only differs slightly from the original chord. If the original chord in a song is G7 (G, B, D, F), the tritone substitution would be D7 (D, F, A, C). Note that the 3rd and 7th notes of the G7 chord are found in the D7 chord (albeit with a change of role

Diatonic function

In tonal music theory, a diatonic function is the specific, recognized role of each of the 7 notes and their chords in relation to the diatonic key...

). The tritone substitution is widely used for V7 chords in the popular jazz chord progression "ii-V-I". In the key of C, this progression is "d minor, G7, C Major". With tritone substitution, this progression would become "d minor, D7, C Major," which contains chromatic root movement. When performed by the bass player, this chromatic root movement creates a smooth-sounding progression. "Tritone substitutions and altered dominants are nearly identical...Good improvisers will liberally sprinkle their solos with both devices. A simple comparison of the notes generally used with the given chord [notation] and the notes used in tri-tone substitution or altered dominants will reveal a rather stunning contrast, and could cause the unknowledgeable analyzer to suspect errors. ...(the distinction between the two [tri-tone substitution and altered dominant] is usually a moot point).".
Tonic substitution is the use of chords which sound very similar to the tonic chord (or I chord) in place of the tonic. In major keys, the chords iii and vi are often substituted for the I chord, to add interest. In the key of C Major, the I Major 7 chord is "C, E, G, B" the iii chord ("III-7") is E minor 7 ("E, G, B, D") and the vi minor 7 chord is A minor 7 ("A, C, E, G"). Both of the tonic substitute chords use notes from the tonic chord, which means that they will usually support a melody that was originally designed for the tonic (I) chord.
The relative major/minor substitution shares two common tones and is so called because it involves the relation between major and minor keys with the same key signatures, such as C major and A minor.
The augmented triad

Augmented triad

In music, an augmented triad is a triad, or chord, consisting of two major thirds . The term augmented triad arises from an augmented triad being a three note chord, or triad, whose top note is raised, or augmented...

 on the fifth scale degree may be used as a substitute dominant, and may also be considered as III⁺, for example in C: V⁺ = G-B-D, III⁺ = E-G-B, and since in every key: D = E.
The chord a minor third above, VII⁷, may be substituted for the dominant, and may be preceded by its ii: vi⁷. Due to common use the two chords of the backdoor progression

Backdoor progression

In jazz and jazz harmony, the chord progression from iv7 to VII7 to I has been nicknamed the backdoor progression or the backdoor ii-V. This name derives from an assumption that the normal progression to the tonic, the ii-V-I turnaround is, by inference, the front door...

 (IV⁷-VII⁷) may be substituted for the dominant chord. In C major the dominant would be G7: GBDF, sharing two common tones with B7: BDFA. A and F serve as upper leading-tones back to G and E, respectively, rather than B and F serving as the lower and upper leading-tones to C and E.

Application

In jazz, chord substitutions can be applied by composer

Composer

A composer is a person who creates music, either by musical notation or oral tradition, for interpretation and performance, or through direct manipulation of sonic material through electronic media...

s, arranger

Arranger

In investment banking, an arranger is a provider of funds in the syndication of a debt. They are entitled to syndicate the loan or bond issue, and may be referred to as the "lead underwriter". This is because this entity bears the risk of being able to sell the underlying securities/debt or the...

s, or performers. Composers may use chord substitutions when they are basing a new jazz tune on an existing chord progression from an old jazz standard

Jazz standard

Jazz standards are musical compositions which are an important part of the musical repertoire of jazz musicians, in that they are widely known, performed, and recorded by jazz musicians, and widely known by listeners. There is no definitive list of jazz standards, and the list of songs deemed to be...

 or a song from a musical; arrangers for a big band

Big band

A big band is a type of musical ensemble associated with jazz and the Swing Era typically consisting of rhythm, brass, and woodwind instruments totaling approximately twelve to twenty-five musicians...

 or jazz orchestra may use chord substitutions in their arrangement of a tune, to add harmonic interest or give a different "feel" to a song; and instrumentalists may use chord substitutions in their performance of a song.

Jazz "comping

Comping

Comping is a term used in jazz music to describe the chords, rhythms, and countermelodies that keyboard players or guitar players use to support a jazz musician's improvised solo or melody lines....

" instruments (piano, guitar, organ, vibes) often use chord substitution to add harmonic interest to a jazz tune with slow harmonic change. For example, the jazz standard chord progression of "rhythm changes

Rhythm changes

In jazz and jazz harmony, "rhythm changes" refers to the chord progression occurring in George Gershwin's song "I Got Rhythm". This pattern, which forms the basis of countless jazz compositions, was popular with swing-era musicians: It is found in "Shoeshine Boy" and "Cotton Tail" written by...

" uses a simple eight bar chord progression in the bridge which uses the chords III7, VI7, II7, V7; in the key of B, these chords are D7, G7, C7, and F7 (each for two bars). A jazz guitarist might add a "ii-V7" aspect to each chord, which would make the progression: "a minor, D7, d minor, G7, g minor, C7, c minor, F7. Alternatively, tritone substitutions could be applied to the progression.
Theoretically, any chord can be substituted for any other chord as long as the new chord supports the melody, but in practice only a few options will sound musically and stylistically appropriate for a given song. This is a technique employed in music such as bebop

Bebop

Bebop differed drastically from the straightforward compositions of the swing era, and was instead characterized by fast tempos, asymmetrical phrasing, intricate melodies, and rhythm sections that expanded on their role as tempo-keepers...

 or fusion

Jazz fusion

Jazz fusion is a musical fusion genre that developed from mixing funk and R&B rhythms and the amplification and electronic effects of rock, complex time signatures derived from non-Western music and extended, typically instrumental compositions with a jazz approach to lengthy group improvisations,...

 to give a music piece more sophisticated harmony, or to create a new-sounding re-harmonization of an old jazz standard

Jazz standard

Jazz standards are musical compositions which are an important part of the musical repertoire of jazz musicians, in that they are widely known, performed, and recorded by jazz musicians, and widely known by listeners. There is no definitive list of jazz standards, and the list of songs deemed to be...

.

Jazz soloists and improvisers also use chord substitutions to help them add interest to their improvised solos. Jazz soloing instruments that can play chords, such as jazz guitar, piano, and organ players may use substitute chords to develop a chord solo over an existing jazz tune with slow-moving harmonies. Also, jazz improvisers may use chord substitution as a mental framework to help them create more interesting-sounding solos. For example, a saxophonist playing an improvised solo over the basic "rhythm changes" bridge (in B, this is "D7, G7, C7, and F7", each for two bars) might think of a more complex progression that uses substitute chords (e.g., "a minor, D7, d minor, G7, g minor, C7, c minor, F7). In doing so, this will imply the substitute chords over the original progression, which will add interest for the listeners.

Sources