To play guitar chords well, a musician needs to understand how chords are built based on musical scales. Major chords use the first, third, and fifth notes of the scale, so a C major chord contains C, E, and G. Other chords like minor, diminished and augmented are also built using set musical intervals from the root note. As a guitarist, you may double notes in a chord based on your strumming pattern, but the notes included should match the required chord structure.