The objects of the previous chapter were graphics aimed at showing things that cannot be seen. I argued that such graphics are based on rules that are largely shared and accepted, even though they have never been formalized or imposed. The same fact is true of the graphics that are the subject of chapter 7-those created to represent time. Representations of time form a vast and varied body of graphic productions. But despite its diversity, the theme of time posseses an underlying unity and a strong structural homogeneity. As in the preceding chapter, I claim that such unity stems not only from our own cultural history, but also from constraints that are cognitive and perceptual in nature.