The common topo is known as a 7.5 minute map, because it depicts 7.5 minutes of latitude and 7.5 minutes of longitude. Another name used often is a 'quad', referring to the quadrangular shape of the map. This often results in mis-representation of current features. The contour lines shown on the topo are the intersection of the land with any of those planes, so that the outlines scribed by the lines provide a bird's-eye representation of the terrain. The distance between adjacent contour lines is called the contour interval.
Some maps use forty-foot contour intervals, while others use twenty.
There is a phrase on the bottom margin of the map which states 'contour interval xx feet'. For the remainder of this series, we'll assume a contour interval of twenty feet. If an area has many contour lines, the land rises upward more steeply compared to a place where there are few. With practice on a topo, one can become accomplished at envisioning the type of terrain depicted, recognizing features such as cliffs, ridges, valleys, arroyos, and saddles. Black is also used for county lines, labels on map features, and other items.