It is interesting to note here that the chord progression used for this
song is exactly the same as the chord progression used in the
Eagles classic "Desperado". Play it, you'll see (obviously, the duration
of the chords is somewhat altered).
Here are a few vitally important music theory concepts
to mediate upon, and fix firmly in your mind.
1. Every major scale has 7 notes.
2. Every note gets two other notes added to it, stacked
on top of it, to become a "chord". All the notes are 3 letters away from
each other. (G-B-D; A-C-E; B-F#-D, etc.)
3. This results in 7 chords that constitute the chords
of that key.
4. The 1st, 4th, and 5th chords are all "major" chords.
They are called the "principal chords of the key" in Mel Bay.
5. The 2nd, 3rd, and 6th notes are "minor" chords. (The
7th is diminished).
6. The 5th chord is special. It is called the "dominant
chord". It usually has an extra note added to it, the 7th, which means
the 7th letter from the root note. In the key of G, that chord is D7.
The "7th" added to it is the note C, 7 from the root note "D".
7. This 5th chord with the added 7th is called "The
Dominant 7th Chord". It is the most important chord in the key. It is
special, because each key has only one dominant 7th chord. In other
words, every key has two major chords, or two major 7th chords, or 3
minor 7th chords, but there is only one dominant 7th chord for every
key. This means we can uniquely identify a key by identifying its
dominant 7th chord.
8. This means every time I see a dominant 7th chord, I
can tell what key I am in. if the song contains a number of
dominant 7th chords (anytime a chord just has a letter and a 7 after it,
it is a dominant 7th chord, such as G7, D7, A7, but not Am7, or Dmaj7),
it is actually changing key every time a new dominant 7th chord is used!
This has big implications if you are improvising. It
means you need to play out of a different major scale for each one of
those keys. You must play out of the major scale from the key that has
that chord as its V chord. So, for a G7, play C major, D7, play G major,