Watch the Harmonized Scale online guitar lesson by Jeff Scheetz from Street Theory for Guitarists
In order to find out what chords are in what key, we need to create what is called the harmonized scale. We also call this the "chord scale" because we're literally playing a scale based off of chords instead of notes. We build a chord on each note of the major scale by stacking intervals of a third. This gives us all the chords in that key.