In jazz music, and particularly in “Be-Bop”, there is a harmonic cadence called the “II-V-I”. This is by far the most widely used cadence. It is found in virtually all standards. But what is an II-V-I?

They are just degrees in Roman numerals of the major scale harmonisation (intervals of thirds).

The II-V-I cadence in the case of C major is:

If we play the C major scale on this cadence, no note will be wrong.

Conversely, the licks that we play will seem bland or boring.

It is the “dissonance” or perceived friction that gives the jazzy licks their character. But where does it come from?

Well, it comes from the fact that Jazz musicians change the “V”. This means that on a “V7″ chord, in our case, G 7(alt), only the essentials are kept: the tonic (G), third (B) and seventh (F). The other notes: ninth, fifth, sixth (or thirteenth) are played a semi-tone above (#) or below (b) depending on the tension they want to create.

I will show you some ways of changing the “V7″ to create tension but also to choose a distinctive note for the final C maj7 chord. I arbitrarily chose the third (E) for the final note because I think it best characterises the chord. However, we can of course choose any note from the chord.

In jazz and jazz harmony, the term altered chord, notated as an alt chord, refers to a dominant chord, “in which neither the fifth nor the ninth appears unaltered” – namely, where the 5th and the 9th are raised or lowered by a single semitone, or omitted. Altered chords are thus constructed using the following notes, some of which may be omitted:

• root
• 3
• ♭5 and/or ♯5
• ♭7
• ♭9 and/or ♯9

Altered chords may include both a flatted and sharpened form of the altered fifth or ninth, e.g. G7♭5♯5♭9; however, it is more common to use only one such alteration per tone, e.g. G7♭5♭9, G7♭5♯9, G7♯5♭9, or G7♯5♯9.

The choice of inversion, or the omission of certain tones within the chord (e.g. omitting the root, common in guitar harmony), can lead to many different possible colorings, substitutions, and enharmonic equivalents. Altered chords are ambiguous harmonically, and may play a variety of roles, depending on such factors as voicing, modulation, and voice leading.

The altered chord’s harmony is built off the altered scale, which includes all the alterations shown in the chord elements above:

• root
• ♭9 (=♭2)
• ♯9 (=♯2 or ♭3)
• 3
• ♯11 (=♯4 or ♭5)
• ♭13 (=♯5)
• ♭7

Altered chords can be analyzed as a kind of tritone substitution (♭5 substitution). Thus the alt chord on a given root is the same as the 7♯11 chord on the root a tritone away (e.g., G7alt is the same as D♭7♯11.

Altered chords are commonly substituted for regular dominant V chords in ii-V-I progressions, most commonly in minor harmony leading to an i7 (tonic minor 7th) chord.

More generally in jazz, the terms altered chord and altered tone also refer to the family of chords that involve ♭9 and ♭5 voicing, as well as to certain other chords with related ambiguous harmony. Thus the “7♭9 chord” (e.g. G7♭9) is used in the context of a dominant resolution to a major tonic, which is typically voiced with a ♮13 rather than the ♭13 of the alt chord. When voiced with a ♮13, jazz musicians typically play the half-step/whole-step diminished scale over the ♭9 chord (e.g. G, A♭, B♭, B, C♯, D, E, F over G7♭9).

Note that in chord substitution and comping, a 7♭9 is often used to replace a diminished chord, for which it may be the more “correct” substitution due to its incorporation of an appropriate root tone. Thus, in a progression where a diminished chord is written in place of a G7 chord, i.e. where the dominant chord is replaced by an A♭-dim (A♭-C♭-E = G♯-B-D), D-dim (D-F-A♭), B-dim (B-D-F), or F-dim (F-A♭-C♭ = F-G♯-B)), a G7♭9 is often played instead. G7♭9 (G-B-D-F-♭A) contains the same notes as any of these diminished chords with an added G root.