when there is an order to the values, but it is not a "more vs. less" order, such as seasonality. It is frequently used by non-cartographers in situations where other color progressions would be much more effective. A Qualitative progression uses a scattered set of hues in no particular order, with no intended difference in value. This is most commonly used with nominal categories in a qualitative choropleth map, such as "most prevalent religion." Bivariate choropleth maps It is possible to represent two (and sometimes three) variables simultaneously on a single choropleth map by representing each with a single-hue progression and blending the colors of each district. This technique was first published by the U.S. Census Bureau in the 1970s, and has been used many times since, to varying degrees of success. This technique is generally used to visualize the correlation and contrast between two variables hypothesized to be closely related, such as educational attainment and income. Contrasting but not complementary colors are generally used, so that their combination is intuitively recognized as "between" the two original colors, such as red+blue=purple. The technique works best when the geography of the variable has a high degree of spatial autocorrelation, so that there are large regions of similar colors with gradual changes between them; otherwise the map can look like a confusing mix of random colors. They have been found to be more easily used if the map includes a carefully designed legend and an explanation of the technique. Legend A choropleth map uses ad hoc symbols to represent the mapped variable. While the general strategy may be intuitive if a color progression is chosen that reflects the proper order, map readers cannot decipher the actual value of each district without a legend. A typical choropleth legend for a classed choropleth map includes a series of sample patches of the symbol for each class, with a text description of the corresponding range of values. On an unclassed choropleth map, it is common for the legend to show a smooth color gradient between the minimum and maximum values, with two or more points along it labeled with corresponding values. An alternative approach is the histogram legend, which includes a histogram showing the frequency distribution of the mapped variable (i.e., the number of districts in each class). Each class may be represented by a single bar with its width determined by its minimum and