So, the next time you see a C major chord or an A minor chord, you might think to yourself, "hmmm.... I wonder if this song comes from the C major scale..." For example, if you saw the chord progression:
C - Am - F - G
it'd probably be a safe bet that this tune comes from the C major scale. Why? Because from the last page, you know that the chords derived from the C major scale are:
C - Dm - Em - F - G - Am - Bdim
Is every one of the chords in the first progression contained in the list above? Why, yes it is.
To further confuse things, music theory assigns Roman Numerals to chords derived from scales based upon the key of the song. In our case, the key is C, so everything is relative to C. The roman numerals for our 7 chords derived from C major are:
| Chord | C | Dm | Em | F | G | Am | Bdim |
|---|
| Roman Num | I | II | III | IV | V | VI | VII |
|---|
Why would someone do this? Well, right now, we're looking at everything in the key of C, but it would nice if we could generalize it to any key, like G or Bb. If we look at our first progression:
C - Am - F - G
We can describe it by its Roman Numerals as:
I - VI - IV - V
Why is this important? Is there any reference to C major in our Roman Numeral description? I think not. This means that we can use the Roman Numeral system to describe a chord progression such that it can be played in any key, and not just C major. Sure, in C major, our progression is:
| C Major | C | Am | F | G |
|---|
| Roman Num | I | VI | IV | V |
|---|
but in G major, it's:
| G Major | G | Em | C | D |
|---|
| Roman Num | I | VI | IV | V |
|---|
Thus, Roman Numerals allow you to generalize a chord progression so that you can play it in any key. Here's how these two progressions sound:
