Geometric Shapes because we do not perceive geometric shapes exactly as they are drawn. We tend to magnify horizontal lines and reduce vertical. So a square needs to be slightly increased in height to appear square and lines will appear thicker if horizontal rather than vertical.
Page symmetry because we tend to see the center of a page as being a little above the actual center In graphic design this is known as the optical center–and bottom page margins tend to be increased by 50% to compensate.
