Hey Friend,
Before I get back to discussing more usable chords, let's take a quick look at Diminished chords, which are the seventh chord in the "harmonic scale."
We use the Roman numeral vii° in a musical score to denote a diminished chord..
The Diminished chord is the one that doesn't sound quite right compared to the other chords in the series. The reason for this is its five note is flattened, meaning is is a minor fifth note from the scale.
We don't see this happening with any other chord in a major scale, and this gives the diminished chord a very "off" sound.
In major and minor chords, we're used to the fifth note supporting the root note (tonic). The tonic and fifth (also known as the 'perfect fifth' interval) are very close to each other in terms of the number of vibrations it takes to make the sound. When physicists talk about sound waves and the difference between two waves, it's typically shown as a ratio.
For example, with an octave interval, we play two notes. The two notes are the same, one being a low pitch and the other being a higher pitch, and have a ratio of 2:1. This means the higher pitched wave vibrates two times to make sound for every one time the lower pitched wave occurs.
Practically, play any open string on your guitar. The string will vibrate between the guitar nut and the bridge.
When you pluck the string, the string vibrate ten times in one second to produce the sound you hear (it's actually a lot more vibrations, but we'll keep the math simple). Now put a finger at the twelfth fret on the same strirng and pluck it. This is the octave note, so the string now vibrates 20 times between your finger and the bridge.
With the fifth note (perfect fifth), the ratio is 3:2. That's saying the higher pitched note (the fifth) is vibrating three times compared to the lower pitched note (the tonic or one note) vibrating twice.
If you play the open A note on your guitar (fifth string), and then you play the E note on the seventh fret of the same string, you've basically cut off one third of the string with your finger on the seventh fret. So an open A string vibrates ten times for the A note, and it would vibrate 15 times between your finger and the bridge for the E note at the seventh fret.
Okay, a long explanation to give you a little background on the physics, but this is why the perfect fifth note works so well with the tonic to create strong sounding chords - the ratios are close, so they sound very complementary.
All major and minor chords have the benefit of using the tonic and perfect fifth in their construction. But the poor diminished chord has to settle for a "minor flatted-fifth" note. And this note throws the whole chord sound way off.
They call the sound "dissonant," and it creates a feeling of discomfort. I'd almost call it anxiety, because the feel urges you to find some comfort. The diminished chord practically pushes you to play the root major chord right away, just to feel the relief.
I think of it like being on a long, tiring trip. You've been driving for hours in sleet and rain, and you're exhausted. The miles tick away, 200, 160, 90, 55... You feel like you'll never get there, the road just keeps going. And yet you know there's a hot shower and a soft bed waiting for you at home. That thought compels you to keep going until you final pull in the driveway.
We'll take a closer look at the breakdown of diminished chords tomorrow.
As you can imagine, you mess up any power chord by changing the fifth note. But I have a surprise for you tomorrow - a song that effectively used this diminished feel to rock it out. It's also cool in that it uses an inversion of the power chord for its iconic intro.
Peace~
Dave | 
