HARMONY
In Hexagram Theory, lines that occupy parallel positions in the lower and upper trigrams are in harmony with each other. They represent a common theme. These harmonies are the 1st and 4th lines (which represent a foundation line for their respective trigrams), the 2nd and 5th lines (which represent lines of projection) and the 3rd and 6th lines (which represent lines of mutation). (see Dissonance and Resonance)