h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Thanks for posting this very nice introduction. I would like to mention three points.

1. Just for ease, what you call the 7th chord is called the dominant 7th chord. Also, the minor 7th chord is the dominant 7 with a flatted 3rd. The major 7 is as you describe. There is also a half-diminished 7 chord, which is the diminished triad with the dominant 7 tone, and the fully diminished chord where all tones are separated by a minor third. I don't mean to be picky. It just facilitates discussion. It is true that on most musical charts, the C7, etc. is understood to be the dominant 7th.

2. I believe that the term altered is a little less restrictive than adding only the +5 to the dominant 7 chord. Correct me if I'm mistaken, but the altered chord is actually based on the altered scale for whatever root names it. In the altered scale, all notes, save the root, are "flatted." This scale is also called the diminished-whole tone scale, because the first four notes are the same as the diminished scale, and all intervals above the fourth tone are whole-tone. (The whole-tone scale is a six six tone scale in which all intervals are whole tones. Thelonius Monk's "Mysterioso" is a great example of a whole-tone tune.) It is also called the super locrian mode. Finally, it is the melodic minor scale in disguise. What is interesting is that the two strongest chord tones in the dominant 7th chord are still present in the altered scale because the flatted 4th is actually the third in the diatonic scale. While the +5 (really the flatted 6) can be part of the chord, it can be absent, while the flat 9 and sharp 9 can be present. Using the alt. designation actually gives the player a great deal of freedom within the altered scale to use different color tones.

3. The first progression you note is actually the set of natural triads within any diatonic scale. This is, indeed very useful to remember. You can also point out that there is a natural set of 7th chords.

Imaj7 IImin7 IIImin7 IVmaj7 V7 VImin7 VII(half-dim)7

One reason this can be extremely useful is one is often asked to transpose a chart on the spot, especially with singers. If you learn to look at a chart as a set of changes (chords) related to the key signature, and if the singer actually knows what key they sing the tune in (not always the case), and if you have done significant shedding so that you actually know all of the chords in all twelve keys, then transposition is pretty easy. That last "if" is the big one.