The concept was sort of new to me too, harmony has always come naturally to me. But to folk new to singing harmony, it seemed to make sense. In a song written in C. you can pretty much sing a G (with C chord or G chord) You might drop down to an F vocally (with G7 chord) or up to an A vocally (with the F chord) This was given only as a starting place for a person whe has never known how to sing harmony.