My favorite two chord song? Clementine! I used to use this as a "first song" for guitarists, teach D and the simple A7 chord that only uses two fingers. and sing the chorus over and over again with instructions "change chords on 'tine'" I quote another one of my heroes, Willard Gibbs-- "The function of theory is to find that vantage point from which experimental observations have the simplest explanation." Wyo, you probably have accumulated a large number of songs for which you have learned the chords by rote, and now you are asking yourself "is there some logic behind all this?" GREAT question, which is how this wonderful thread got started-- and I hope you can see some of the logic and simplifying principles (especially the circle of fifths concept) that helps it start to make sense. After that it can get more complicated again, but the goal is simplicity!