and patterns with minimal reference to the legend. A second general guideline, at least for classified maps, is that the colors should be easily distinguishable, so the colors on the map can be unambiguously matched to those in the legend to determine the represented values. This requirement limits the number of classes that can be included; for shades of gray, tests have shown that when value alone is used (e.g., light to dark, whether gray or any single hue), it is difficult to practically use more than seven classes. If differences in hue and/or saturation are incorporated, that limit increases significantly to as many as 10-12 classes. The need for color discrimination is further impacted by color vision deficiencies; for example, color schemes that use red and green to distinguish values will not be useful for a significant portion of the population. The most common types of color progressions used in choropleth (and other thematic) maps include: A Sequential progression represents variable values as color value A Grayscale progression uses only shades of gray. A Single-hue progression fades from a dark shade of the chosen color (or gray) to a very light or white shade of relatively the same hue. This is a common method used to map magnitude. The darkest hue represents the greatest number in the data set and the lightest shade representing the least number. A Partial-spectral progression uses a limited range of hues to add more contrast to the value contrast, enabling a larger number of classes to be used. Yellow is commonly used for the lighter end of the progression due to its natural apparent lightness. Common hue ranges are yellow-green-blue and yellow-orange-red. A Divergent or Bi-polar progression is essentially two sequential color progressions (of the types above) joined with a common light color or white. They are normally used to represent positive and negative values or divergence from a central tendency, such as the mean of the variable being mapped. For example, a typical progression when mapping temperatures is from dark blue (for cold) to dark red (for hot) with white in the middle. These are often used when the two extremes are given value judgements, such as showing the "good" end as green and the "bad" end as red. A Spectral progression uses a wide range of hues (possibly the entire color wheel) without intended differences in value. This is most commonly used