I know this fella drives some of you nuts..BUT he does explain the workings of music theory in the most thorough and complete fashion.
These chords are very common in all types of music, including our three strands: Western Classical Tradition, World Music and Popular Music. They are the building blocks of The Blues Rock’n’roll and are still commonly used in pop music today.
Note the use of ROMAN NUMERALS to identify chords. This system is used so that we can stop talking about “C, F and G” and discuss harmony without always relating to specific keys. If this system is new to you, have a look at this for explanation of the Roman Numerals in music.