Altered Dominant Chords

Q: I have been going through your site and it is great. I like all of the information you have provided. One thing that still confuses me is what is an altered chord. I see this some time. G7 alt. What does that mean? How do I know what the extra note(s) that need to be added to make this an altered chord? Is there a way to tell what note(s) need to be added based on the key signature? I am confused.

A: Thanks! Glad you like the site and the lessons. You can find most of what you need in two lessons on my site: The Altered Scale and Dominant Chords but let me give you a simple explanation first:

Alt is an abreviation for altered which is refering to the altered scale (the 7th mode of melodic minor). Always keep in mind, every chord has a scale from which it is built. The G altered scale looks like this: G-Ab-Bb-B-C#-D#-F and the intervals: 1-b9-#9-3-b5-#5-b7 (pay close attention to the 5ths and 9ths).

I imagine that the standard 7 chord eventually gave away to a 7b9 chord which has more tension. It is a pretty safe bet that the 7b9 chord was harmonized from the 5th mode of the harmonic minor scale (sometimes refered to as the phrygian dominant scale). Using G (same as C harmonic minor) as an example: G-Ab-B-C-D-Eb-F or 1-b9-3-4-5-b6-b7. See how this scale makes a G7b9 chord?

Eventually musicians looking for even more tension would turn to the altered mode from the melodic minor scale to get some great sounding dominant chords (after all, besides the b9, it also contains a #9, b5 and #5). A G(alt) chord would be any G7 chord with any two (or more) of the altered intervals in it (b5,#5,b9,#9). How do you know which ones to stick on top? You could just pick the ones that you like. For example, I personally like the 7(#5,#9) chord. Sometimes the melody of the song will give you a hint as well. If the melody note in the song is a #9, it would probably be a good idea to play an altered chord with a #9 in it as well.

The altered chord is almost always a "V" chord going to the "I" chord, as in G7(#5,#9) - C maj7. But there are examples where this is not the case.