Now that we know how to construct a major scale using its structure we can begin building chords from the notes that scale contains. This is going to be rediculously simple. All you need is the 1st, 3rd and 5th notes of the scale.
C Major chord? C - E - G. F Major Chord? F - A - C. G Major chord? G - B - D.
But there's a catch... Chords are structured based upon intervals too, and the structure for a Major chord is:
4, 3 (in semitones) or 2, 1 1/2 (in whole steps)
So, for instance, a D Major chord would be D - F# - A and an A Major chord would be A - C# - E Needless to say, these chords do not fit in a C Major scale
To make them fit we must flat the 3rd note, in the case of D, E and A. In the case of B we must also flat the 5th. Since the intervals between the notes of these chords no longer fit the structure of major chords we end up with other chords.
The D, E and A become minor chords and are noted as Dm, Em and Am respectively. The B becomes a B diminished - Bdim (not shown).
Since A minor is the relative minor of the C Major scale these same chords fit right in.
Using these structures we can adapt them to any key by looking at the Circle of Fifths.
