User:Rictus/Jazz theory

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Jazz theory is a branch of music theory that describes the harmonic and melodic structures of jazz music. As in traditional classical music theory, jazz theory is used to analyze the structure of jazz compositions. However, it also serves to guide the choices musicians make while improvising over compositions in the jazz idiom. Thus, jazz theory tends to be more pragmatic than classical theory; it places less emphasis on rigorous analysis, and more emphasis on finding ways to organize the vast array of harmonic and melodic possibilities available during improvisation.

Because of this pragmatic aspect, different musicians often create their own theories of jazz that reflect their own musical styles and ways of organizing information. Indeed, musicians like Steve Coleman have developed highly idiosyncratic theories of harmonic and melodic structure that make little reference to standard jazz theory, and free jazz players may or may not be driven by theory at all. However, there is a common body of theory, stemming from the bebop and post-bop eras, that is generally accepted by most jazz musicians; while not all musicians think alike, they can typically communicate musical ideas by reference to standard jazz theory.

Overview
The form of a jazz tune generally consists of a melody (known as the head) played over a chord progression (known as the changes). When performing a tune, the musicians typically play the head first, then take turns improvising over the changes, then play the head again. As they improvise, they often take liberties with the changes, by adding, removing, or substituting chords on the fly, and choosing notes from different scales to play over a given chord.

The changes generally consist of smaller progressions of related chords. Each smaller progression usually implies a specific key center, much as in classical music. However, key centers often shift quickly, and to unrelated keys, many times within a tune, and individual chord progressions do not necessarily resolve as expected. In fact, in many post-bop tunes, there often is no single overall "key".

Most chords in jazz tunes are seventh chords of various types, often with one or more notes altered. However, a chord is not defined only in terms of the notes directly in the chord. Instead, a given chord is thought of as built from a particular scale associated with that chord. The scale associated with a chord provides a set of notes that can be played over the chord during melodic improvisation, and can also be added to the voicing as extensions of the basic chord.

Jazz notation
Although much of jazz notation and terminology follows standard convention from classical theory, it is somewhat less formal, and deviates from classical notation in a few common ways:


 * As in classical music theory, chord progressions in jazz are described using Roman numerals to indicate how the roots of the chords in the progression relate to a given key center. For example, in the key of C, a I chord has C as the root, a II chord has D as the root, etc.  While classical notation uses lowercase for minor chords, jazz musicians typically always write the numerals in uppercase, appending chord symbols where necessary (e.g. "IIm7" instead of "ii7").


 * Jazz musicians use the term "voicing" to describe a choice of notes and inversion used to play a given chord. For example, a pianist might "voice" Dm7 by playing F, A, C, and E (the 3rd, 5th, 7th, and 9th), or G7 by playing F, A, B, E (the 7th, 9th, 3rd, and 13th).  Chordal instruments often omit the roots of chords when voicing them, because the root is typically played by the bass player, or is strongly implied by the voicing.


 * When describing intervals from the root of a chord or scale, jazz musicians use "flat" and "sharp" instead of "augmented" and "diminished". For example, the diminished fifth is "flat five" (notated b5), and the augmented ninth is "sharp nine" (notated #9).  They also use "flat" to describe the minor ninth and thirteenth ("flat nine/b9", "flat thirteen/b13").


 * When describing the even scale degrees relative to the root of a chord (2, 4, and 6), jazz musicians often use the interval in the second octave, rather than the basic interval in the first octave. Thus, a jazz musician will almost always refer to a "ninth" instead of a "second", and they tend to use "eleventh" and "thirteenth" for m7 and 7 chords.  (They do tend to use "fourth" and "sixth" for maj7 and mmaj7 chords, most likely because of the different functions that these tones play in such chords.)  They will even describe scales in these terms; for example, when describing the scale that goes over an alt chord (described below), instead of saying "root, minor second, augmented second, major third, augmented fourth, minor sixth, minor seventh", a jazz musician is likely to describe it as "root, b9, #9, 3, #11, b13, b7".  This is because jazz musicians think of scale degrees and chord extensions interchangeably, as described in the next section.

Chords and scales
As mentioned above, most chords in jazz tunes are seventh chords. The basic seventh chords most commonly used in jazz are:
 * the minor seventh chord, usually written "m7", consisting of the root, minor third, fifth, and minor seventh. For example, Dm7 contains the notes D, F, A, and C.
 * the dominant seventh chord, usually written "7", consisting of the root, major third, fifth, and minor seventh. For example, G7 contains the notes G, B, D, and F.
 * the major seventh chord, usually written "maj7", consisting of the root, major third, fifth, and major seventh. For example, Cmaj7 contains the notes C, E, G, and B.  On lead sheets, the "maj7" is often omitted.
 * the half-diminished seventh chord, usually written "m7b5" or "ø7", consisting of the root, minor third, diminished fifth, and minor seventh. For example, Cm7b5 contains the notes C, Eb, Gb, and Bb.

Jazz musicians often voice maj7 chords as 6 or 6/9 chords--e.g. C6/9, which consists of C, E, G, A, and D, instead of Cmaj7. The sound of the 6 implies the "tonic" sound of the maj7 without actually playing the maj7. Similarly, mmaj7 chords are often voiced as minor 6 or 6/9 chords.

These chords can all be thought of as built from different modes of the major scale. For example, the specific chords listed above (Dm7, G7, and Cmaj7) are built from the C major scale; if you take alternate notes from the C major scale starting at D, you get the Dm7 chord, and so on. When improvising melodies over these chords, musicians typically use notes from the associated mode; e.g., over Dm7, one would improvise over the D Dorian mode of the C major scale.

When voicing seventh chords on a piano or guitar, musicians often freely add extensions--other notes from the associated scale that are not in the basic chord. The basic extensions are the ninth and sixth (or thirteenth), which can be added to any of the chords above, and the eleventh, which can be added to a minor seventh chord.

In addition to these basic chords, jazz makes extensive use of alterations--chords with tones not in the major scale associated with the basic seventh chord. For example, the C7b9 chord consists of C, E, G, Bb, and Db. In general, alterations are made to the fifth of the chord or to the higher extensions (ninth, eleventh, and thirteenth); the root, third, and seventh of the chord are not usually altered, since doing so would completely change the basic quality of the chord altogether. Because altered chords incorporate non-chord tones, they generally imply scales other than the major scale associated with their unaltered forms. Some chords have multiple possible associated scales; which one a musician chooses to improvise with is a matter of personal choice.

[table of scales] [table of chords] [other scales: blues scale, pentatonic scale, harmonic minor] [avoid notes]

Chord progressions
As mentioned above, the large-scale structure of a jazz tune often contains frequent key modulations. However, at a smaller scale, typical straightahead jazz tunes and standards tend to consist of common "micro-progressions" of two, three, or four related chords.

The most common such micro-progression in jazz is the IIm7-V7-Imaj7 progression, usually abbreviated to II-V-I. II-V progressions that don't resolve to a I are also common, as are V-Is with no preceding II.

A related common progression is known as the "minor II-V-I", which is typically played as either IIm7b5-V7alt-Immaj7 or IIm7b5-V7b9-Immaj7.

Standard tunes that begin and end in the same major or minor key often have a "turnaround" progression at the end. For example, if a tune resolves to a I chord one or two measures before the end of the form, and the beginning of the tune is the same I chord, rather than hold on that I chord for the extra measures, the form will add a progression like VIm7-IIm7-V7 to lead back to the I chord at the beginning of the tune. Many variations on this turnaround exist; see the substitutions section below for examples.

There are also a few common "macro-progressions" that form the entire structure of many jazz tunes. The most common is the 12-bar blues. [12-bar blues, rhythm changes]

[Coltrane changes]

Substitutions and reharmonization
[tritone sub] [other common subs: 6/9 or lydian for maj7, m7 for mmaj7, alterations of dominant, sus, dominant for m7, treating m7 as mmaj7 (tonic I) temporarily even when it turns into a II] [adding/removing II-V or V] [changing roots] [table of possibilities for harmonizing different intervals above root]