Altered chord in the context of "Bebop"

Jazz improvisation is the spontaneous invention of melodic solo lines or accompaniment parts in a performance of jazz music. It is one of the defining elements of jazz. Improvisation is composing on the spot, when a singer or instrumentalist invents melodies and lines over a chord progression played by rhythm section instruments (piano, guitar, double bass) and accompanied by drums. Although blues, rock, and other genres use improvisation, it is done over relatively simple chord progressions which often remain in one key (or closely related keys using the circle of fifths, such as a song in C Major modulating to G Major).

Jazz improvisation is distinguished from this approach by chordal complexity, often with one or more chord changes per bar, altered chords, extended chords, tritone substitution, unusual chords (e.g., augmented chords), and extensive use of ii–V–I progression, all of which typically move through multiple keys within a single song. However, since the release of Kind of Blue by Miles Davis, jazz improvisation has come to include modal harmony and improvisation over static key centers, while the emergence of free jazz has led to a variety of types of improvisation, such as "free blowing", in which soloists improvise freely and ignore the chord changes.

A borrowed chord (also called mode mixture, modal mixture, substituted chord, modal interchange, or mutation) is a chord borrowed from the parallel key (minor or major scale with the same tonic). Borrowed chords are typically used as "color chords", providing harmonic variety through contrasting scale forms, which are major scales and the three forms of minor scales. Chords may also be borrowed from other parallel modes besides the major and minor mode, for example D Dorian with D major. The mixing of the major and minor modes developed in the Baroque period.

Borrowed chords are distinguished from modulation by being brief enough that the tonic is not lost or displaced, and may be considered brief or transitory modulations and may be distinguished from secondary chords as well as altered chords. According to Sheila Romeo, "[t]he borrowed chord suggests the sound of its own mode without actually switching to that mode."

In jazz, a bar-line shift is a technique in which, during improvisation, one plays the chord from the measure before, as an anticipation of a chord, or after the given chord, as a delay, either intentionally or as an "accident."

Bar-line shifts may be caused by a novice having lost their place in the chord progression, but is most often attributable to: "(1)...harmonic generalization, as in the case of playing a II to V7 (+5, +9) progression [II-V-I turnaround] as only a V7 (+5, +9); or (2) the player wanted to play the previous chord (though it has already transpired), but was either pausing momentarily (as in taking a breath), and decides to adopt the 'better later than never' attitude." An example of a "very intentional" bar-line shift may be found on Cannonball Adderley's solo on "So What," "in which he deliberately enters and exits the bridge early, causing considerable tension, since the chord of the A section (D-) is one-half step lower than the chord of the bridge (E♭-)." Other notable performers to use this technique include Charlie Parker, Coleman Hawkins, and Pat Metheny.