One guitar is playing a I IV V progression using ... This would be a bVI chord, which is not part of the major key of D major. This borrowed chord creates a feeling of tension before going back ...

The ‘happiest’ sounding of the modes, the Ionian is identical to the major scale, so it’s highly likely you’re already playing it on guitar ... E and A in the key of A major.

This is used when writing a melody. Each key signature has two possible keys. The major key and the minor key, which will begin on a different note. This is known as the relative minor.