Hopefully you are comfortable with the last two lessons (major scales and triads). In this one, we're really just putting the two together. I'll be using the key of C again, which you'll remember is C - D - E - F - G - A - B - C.
Before we get into this, remember how we discussed the "anatomy" of major, minor, augmented, and diminished triads? Here's the list:
Now, remember how we found a "hidden" chord in the key of C major? It was D minor, which we found by starting on D and skipping notes until we got three (for a triad). Well, it turns out there are SEVEN triads in the key of C major, because we can make a triad starting on every note! These would be:
C E G
D F A
E G B
F A C
G B D
A C E
B D F
I just listed the original key down the left in bold, then skipped notes and found my triads. Now we should figure out what these sound like. Here's a clip of all of them on piano. Try to hear if they're major, minor, or whatever (sorry, I kinda went nuts at the end playing random triads).
Triads in C Major on Piano
If it sounds like you've heard this before, it's because your brain knows what's coming next. Most music you listen to (almost any style) uses the major scale, so it's no surprise that these chords are in your head to stay.
Remember, the stuff you're learning about triads and major scales holds true in ANY key. I'm using the key of C here for simplicity, but you can use any scale you want to make triads! Try this now - just pick any random note on the guitar, make a major scale (using the pattern from the other lesson), and then find all seven triads. You get the same pattern and sounds no matter what key you are in!
Oh, and while you're doing this exercise, try to figure out what the triads are - major, minor? You can do this with the handy chart of triads above.
Now that you can do that (I hope), we're ready to label the chords. The following system is used by all kinds of musicians in lots of different genres to refer to the chords in a key. Let's bring out the Roman numerals:
C E G - Major - I
D F A - Minor - ii
E G B - Minor - iii
F A C - Major - IV
G B D - Major - V
A C E - Minor - vi
B D F - Diminished - vii°
Yay! Notice how the major chords are "capital" Roman numerals and the minor chords are "lowercase". This is for clarity. There's only one diminished chord - the vii°. It sounds sort of odd with all the major and minor chords, but it does "fit" in a way. It's definitely the least used of all the chords in "standard" chord progressions.
You may have noticed that chords like C, Am, G, and F go together quite nicely and make a good progression. They're in a key together, so it makes sense. If we were to make a progression with those chords, we could label it:
I vi V IV
Why go to all this trouble to label chords as Roman numerals? We could have just written the regular chords down.
The Roman numeral system is very useful. Later on, when you get deeper into music theory voodoo, it's helpful to think of chords this way. For instance, you can transpose chord progressions easier once you learn your key signatures (we'll cover all this later).
After a couple years of familiarity with this system, you start to hear how these chords sound compared to other ones in the same key. You start hearing what a IV chord sounds like, for instance. I can hear almost any rock song and tell you what the progression is without even using my guitar. This is not only fun but helps me solo better and to just about anything.
You can communicate with other musicians on this level. Sometimes you might hear someone say, "Play a I vi ii V progression in the key of D" or something similar.
This was a long lesson, but it packed a lot in. I hope you understood it! Keep in mind that this is relatively advanced stuff, and if you understand it you're doing quite well!