Hey Friend,
As we found with the key of C major, all the chords that make up the harmonic progression for the key of G major are constructed using the notes from the key of G. Recall from yesterday the chords of G major:
G Am Bm C D Em F#°
Each one of these chords is constructed of three notes, and each of these three notes can be found in the G major scale. Here's that:
G A B C D E F#
Remember that a chord is considered three notes or more (a triad), and that a major chord uses the 1 3 5 notes from the scale (key). Just to be crystal clear, look here:
1 2 3 4 5 6 7 G A B C D E F#
A G major chord (G) uses notes G, B and D. A B minor (Bm) chord uses B, D and F#. It even works for the F#° chord - F#, A and C.
To be clear, there are other chords we can use as substitutes for chords within a key, and they will have slightly different note combinations (outside the scale) to make the chord correct. This gets done fairly often to keep you interested. Our brains instinctively know when a note is out of place, so we throw in a chord that has notes just outside the key to make the listener perk up.
For example, let's say you wanted to substitute a dominant 7 chord for a minor chord. Look at this:
G A7 C D
Instead of A minor (Am) with notes A, C and E, we substitute in A7 (A dominant 7) with notes A, C#, E and G. While the A7 isn't a primary chord in G major, it still works extremely well within a G major progression.
In fact, I think it's a much more interesting chord than A minor in this context. Your ears can tell A7 doesn't really fit in the harmonic progression of G major, but it still works and it makes us take notice.
But for the basic harmonic progression, the key holds all the notes you need to construct the chords belonging to the key.
We'll review the G major chord tomorrow. See you then.
Peace~
Dave | 
