Occasionally, I get a question about altered chords. What is an altered chord, and when do you use it? Where do they come from?
First, we need to have a fundamental understanding of the way harmony works. All harmony (and melody for that matter) in the western world comes from the major scale. This set of notes is a combination of whole steps and half steps. The half steps are where much of the tension comes from that provides the "action" of the story in a piece of music. A V7 chord has, for hundreds of years, represented the most prominent action or tension sound, and this resolves to the I chord, which sounds like we're back home. The V7 chord contains two of the tension notes: the 7th scale degree and the 4th scale degree. The 7th resolves up a half step to the root of the I chord, and the 4th resolves down a half step to the 3rd of the I chord. This double resolution has formed the backbone of harmony in western music for a long time. However, anything gets old eventually, and composers began to get bored with just diatonic harmony.
And this is where altered chords come in. You might think that there's an "altered key" where all the "altered chords" originate, and this music sounds really strange and exciting, but that's not really the way it works. There isn't really a convenient diatonic "altered" harmonized scale that all "altered" chords come from. First of all, the term "altered" is a little vague; it has different meanings to different people. I think it would be safe to say that a generic definition of the term might be something like: An altered chord is a chord where one or more of the tones is altered from what they would naturally be in the current key center.
So, in the key of C, any chord that used a note other than C, D, E, F, G, A, B would be considered "altered." Probably the most common type of chords in jazz that tend to be altered are dominant seventh chords. Usually, adding "altered" notes from outside the key center gives an increased sound of dissonance, which works well with the dominant since this sound usually functions as a dissonance that gets resolved anyway. So adding altered notes increases the tension, making the resolution that much more satisfying. Typically, you wouldn't alter the fundamental chord tones of Root, 3rd or 7th; this would make the chord sound like something else other than a dominant chord. So, the notes that tend to get altered are the upper extensions plus the 5th. The 5th is a more "neutral"-sounding note that doesn't add much to the fundamental sound of the chord, so it's easy to change it without taking away from the chord's fundamental identity. It just so happens that there are two scales, the so-called "altered scale" (which is the seventh mode of melodic minor) and the half-whole diminished scale, that contain all essential notes of a dom 7 chord, plus many of the possible altered upper extensions. In C again, this would be:
G, Ab, A#/Bb, B, C#, Eb, F
This contains the basic essential chord tones of G7:
plus the upper extensions of:
Notice that there are some enharmonic equivalents here; the A# is the same note as Bb, the b3 of G. A b3 on a major chord tends to sound like a "blue note." Also, the C# is the same note as Db, the b5 of G, and the Eb is the same note as D#, the #5 of G. So any combination of these notes would jibe with the altered scale.
Note, also, that the notes in the G altered scale are the same as the notes in the Ab melodic minor scale, so as a shortcut to getting your feet wet with this sound, you could think "melodic minor up a half step" when soloing or comping over a V chord.
On to the G half whole diminished scale:
G, Ab, A#, B, C#, D, E, F
Here, we have all 4 chord tones of G7:
plus the upper extensions of:
Notice that a lot of similarities exist between the altered scale sound and the half-whole diminished sound. The most obvious difference is that the half-whole diminished has a natural instead of a flatted 13th. The natural 13th gives what could be described as a slightly brighter sound. Either of the two sounds work nicely for increasing the tension in anticipation of a resolution back to the I chord, though.
Of course, normally in the major scale, there is only one dominant 7th chord: the V chord. Another concept that comes up quite a bit in jazz (and other types of music as well) is that of the secondary dominant. Any major (including the dominant V chord) or minor chord in the key can be preceded by a chord that is the V chord of THAT chord. For example, in C major, the II chord is Dm7. Normally in D major or minor, the V chord is A7. Adding an A7 chord before a Dm7 can increase the tension and the sound of needing to resolve to the Dm7 chord:
| Cmaj7 | | Dm7 | G7 | Cmaj7 |
with secondary dominant of II:
| Cmaj7 | A7 | Dm7 | G7 | Cmaj7 |
To further increase the tension, altered tones could be added to the A7 chord, and these sounds could be thought of as coming from the A altered or A half-whole diminished scales. Note that other scales contain some of the altered notes, but the altered scale and half-whole diminished scale are probably the most common to be associated with V chords and secondary dominants. Also, it's a good idea for beginning-to-intermediate musicians to become comfortable with improvising with the basic chord tones before getting too deeply into altered harmony and substitute scales like the altered and diminished.
Lastly, chords other than the dominant can certainly be altered and often are. Some other common alterations (again, in the key of C) would be:
#11 on the I chord (Cmaj7#11)
This could be thought of as a sound borrowed from the key of G (all natural notes plus an F# is the key of G).
b5 on the II chord (Dm7b5)
This could be thought of as a borrowed sound from the key of C minor. C natural minor will work, as will F melodic minor, which has all 4 chord tones plus upper extensions of 9, 11, and b13.
b3 on the IV chord
This fundamentally changes the sound of the IV chord from major to minor, of course, but creates a nice pull back to the I chord since Ab and F (the b3 and root of F minor) are a half step away respectively from G and E (5th and 3rd of C major). Many scales are possible, but one that produces many common tones with C major is F melodic minor. This will yield an F minor triad with a natural 7, 9, 11, and 13 (the only notes different from C major will be the b3, Ab, and the 11th, Bb.)
Note: a common substitute for the IV minor sound is the bVII dominant sound. If you take the F melodic minor scale, start from Bb (the 4th degree), and harmonize in thirds, you end up with:
This is usually referred to as the "Lydian dominant" sound. Very often in standards, you'll see this chord used in place of a minor IV:
| Fmaj7 | Fm | Cmaj7 |
| Fmaj7 | Bb7 | Cmaj7 |
In this case, the F melodic minor scale forms the basic sound of the second chord, but in the second case, a different root movement gives a slightly different flavor to the progression. In either case, the soloist could use the F melodic minor scale and get the same effect.
These are some of the more common "alterations" of diatonic major scale harmony. Many other possibilities exist, but I hope this helps at least scratch the surface of explaining what musicians are talking about when they mention altered chords.