Maps show locations; in addition, they often show themes. Themes can be ways to conceptualize the Earth, composed of biomes or watersheds, for example, or a series of events, such as a theme of the Napoleonic wars, or a specific variable, such as median age by country or by census tract, or in myriad other ways (Tufte, 1990). Figure 6.1 shows a “thematic” map. In it, a set of data, or theme, has been partitioned into intervals. In theory, there are an infinite number of ways available to partition each theme. In practice, a finite set is typically employed. The method one chooses to partition the data, into mutually exclusive ranges, has a profound impact on how the resulting map appears and how it is interpreted (Monmonier, 1993, 1996).