Edge Makes the Difference

When the tassel hangs freely and covers the border between the halves, few people can see the difference in the color. It is very subtle. It is only when the border is exposed that it becomes apparent.
Edges are important in defining what we see. When the rope tail is lifted, two separate squares become visible, one significantly darker than the other. When the rope tail coveres the edge, the two sides are indistinguishable. The two squares are really identical. There is only a difference at the point where they meet—the edge. Each square is shaded lighter to darker, from left to right. In the middle, the dark edge of one square meets the light edge of the other. The eye concentrates on the boundary and assumes the rest of the square is the same.