Looks at overtones (harmonics) generated by a single plucked string, and their relation to harmony. Shows that the basic chords are generated by these fundamental overtones.
Delves into the dark world of minor harmony. (Faure's 'Pavane' played on harp).
Polyphony - many voices combining to form chords (Mass for Four Voices by William Byrd.)
Which chord to use? Demonstrates by harmonising the Xmas carol "Silent Night" at the piano. Points out that the 24 major and minor triads harmonised practically every tune between A.D. 1200 and A.D. 1600!