A Dominant 9th

M3 + m3 + m3 + M3

Today’s surprise (for me) is courtesy of the A dominant 9th (A9) chord. Actually, it doesn’t really matter the chord, but this is what I was looking at when I had a realization. I’ve known the notes in this chord, played it on the guitar and probably piano, but it never struck me what it was - or, at least, one thing that it is; it’s an A major (A, C#, E) and E minor (E, G, B) played together.

It’s not important that it’s these two chords, but seeing this helps me to understand what this might sound like, even without playing it. I might be able to do silly things like simplify parts on one instrument, if I know others will be covering ‘the rest’ of the chord in the same song. It’s hard to quantify, but connections like this help overall depth of understanding of a topic.

I happened upon this while working on adding interval stacks to the chord detail pages. These interval stacks are a way to see how chords are constructed. A9 is M3 + m3 + m3 + M3; this means it’s a major 3rd, (A → C#), followed by a minor 3rd (C# → E), a second minor 3rd (E → G), and finally another major 3rd (G → B). Major chords are M3 + m3, and minor chords are m3 + M3, hence the C major and E minor.

Even though I’ve known how major and minor chords are constructed, I’ve had the notes listed for chords for a long time, and I have other ways to see how a chord is constructed, seeing this stack allows me to see both chords at once in my mind. I often wondered why this was one way to describe chords, rather than just the scale degree formula, but now I understand.

Anyway, chord pages now give these interval stacks, along with an explanation of how to construct them via either the interval stack or the scale degree formula. I also include a handy diagram, like the one above, on each chord page to help drive the point home.

Check it out today and let me know what you think!