Visual variables, or graphic variables, in Cartographic design, are the ways in which the symbology, or visual appearance, of map elements can be controlled. These visual techniques can be used to create a pleasing aesthetic, convey precise geographic information, and create a visual hierarchy that can be understood by the viewer of the map.
Jacques Bertin, a French cartographer, was the first to develop the concept of visual variables in his book, "Semiologie Graphique," published in 1967. Bertin identified seven main categories of visual variables: position, size, shape, value, color, orientation, and texture. Bertin's original seven variables have been modified and expanded in number by various cartographers and authors (Monmonier, MacEachren, etc.).
The position of map elements is important in cartography. Absolute location on a map cannot be altered, but the position of labels and information can affect the viewer's perception of a map.
The size of a label or symbol is how much space it occupies on a map. Size differences are relatively easy to recognize, making it a useful variable. The size of symbols can convey information, such as a quantitative amount of something, or can be used to attract a viewer's attention (i.e., raising it in the Visual hierarchy. Because geographical features have an actual size on the Earth, this cannot always be controlled, and sometimes works against the wishes of a cartographer. For example, it can be difficult to make a world map in which Russia does not stand out.
A shape is a simple design that is used to symbolize an attribute on a map.  They reference a location with a certain attribute. Different shapes generally correspond with different attributes.  For example if a map of a city has a red cross on it then, most people will assume that there is some sort of medical services provided at that location. Another example would be a star on the map that denotes that a particular location is of some importance to the reader. Some shapes are simple in nature and thus are more abstract, while other shapes are more pictorial and are easy for the reader to comprehend what is trying to be conveyed. Shapes come in many different varieties and should be used in simple symbology as not to confuse the viewer. Generally, shapes are also easily recognized in contrast with one another. Some aspects of shape are inherent to the phenomenon being portrayed and may not be easily manipulable, especially in line and region symbols, such as the shape of a road or a country. In line and region symbols, shape can also play a role as part of a pattern, such as a region filled with tree icons.
Color is the visual perceptual property corresponding in humans to the categories called red, green, blue, and others. Humans generally perceive three aspects of color: hue (the commonly-named colors of the rainbow), saturation (the intensity or brightness of a color), and value (the lightness or darkness of a color). Choropleth maps often use color value to differentiate between characteristics that are being mapped. 
As an aspect of color, value refers to how light or dark an object appears on a map. Value effectively conveys a feeling of "more" and "less," an ordinal measure; this makes it a very useful form of symbology in thematic maps, especially choropleth maps. Value contributes strongly to Visual hierarchy; elements that contrast most with the value of the background tend to stand out most (e.g., black on a white sheet of paper, white on a black computer screen).
Orientation refers to the direction labels and symbols are facing on a map. Although it is not used as often as many of the other visual variables, it can be useful for communicating information about the real-world orientation of features. Common examples include wind direction and the direction in which a spring flows.
Texture refers to the aggregate pattern made up of many individual symbols. For example, a dense network of lines representing streets could collectively convey the concept Urban Area. An evenly spaced lattice of green dots could mean Orchard, while a random distribution of the same green dots could mean Forest.
- Slocum, T. A., McMaster, R. B., Kessler, F. C., & Howard, H. H. (2009). Thematic Cartography and Geovisualization. Upper Saddle River, New Jersey: Pearson Prentice Hall.
- Tyner, J. A. (2010). Principles of map design. New York: The Guilford Press.
- "Shape", GIS Dictionary
- "Visual Variables", Westfaelische Wilhelms Universitaet
- Symbol Basics, "Cartographic Symbols",