Rule 1 - every point of a contour line has the same elevation.
Rule 2 - contour lines separate uphill from downhill.
Rule 3 - contour lines do not touch or cross each other except at a cliff.
Rule 4 - every 5th contour line is darker in color. This is an INDEX contour line.
Rule 5 - Contour lines are closer together in steep terrain and farther apart in flat areas.