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** A ''bivariate [[choropleth map]]'' is the most common type of correlated symbol. Contrasting but not complimentary colors are generally used, so that their combination is intuitively recognized as "between" the two original colors, such as red+blue=purple.<ref name="trumbo1981" /> They have been found to be more easily used if the map includes a carefully designed legend and an explanation of the technique.<ref>{{cite journal |last1=Olson |first1=Judy M. |title=Spectrally encoded two-variable maps |journal=Annals of the Association of American Geographers |date=1981 |volume=71 |issue=2 |pages=259-276}}</ref> A common legend strategy is a two dimensional matrix, divided into smaller boxes where each box represents a unique relationship of the variables.
** A ''multivariate [[Dot distribution map | dot density map]]'' mixes dots of different colors in each district, typically representing separate subgroups of the overall population.<ref name="jenks1953">{{cite journal |last1=Jenks |first1=George F. |title="Pointillism" as a Cartographic Technique |journal=The Professional Geographer |date=1953 |volume=5 |issue=5 |pages=4--6 |doi=10.1111/j.0033-0124.1953.055_4.x}}</ref>
* A ''multivariate symbol map'' represents two or more variables in the same thematic map layer, using distinct [[Visual variable | visual variables]] for each variable.<ref name="slocum2009" />{{rp|337}}<ref name="nelson1996"/> For example, a layer of cities might be symbolized with circles of [[Proportional symbol map|proportional size]] representing its total population, and the hue of each circle representing the predominant source type of its electric power, akin to a nominal [[choropleth map]].
** A ''[[cartogram]]'' distorts the size and shape of a set of districts according to a variable, but does not dictate the symbol used to draw each district. Thus it is common to symbolize them as a [[choropleth map]].
** A ''chart map'' represents each geographic feature with a [[Chart | statistical chart]], often a [[pie chart]] or [[bar chart]], which can include a number of variables. Each chart is usually drawn proportionally to a total, making it a multivariate symbol.
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