An uncrossed line segment,the ends of which are known as nodes. Arcs are bounded on each side by polygons.A simple example would be a city street.The ends of the street (where it meets other streets) are the nodes. On each side of the street we have a different city block (polygons).Because of this special relationship between 0-dimensional nodes, 1-dimensional chains, and 2-dimensional polygons,the arc is the most fundamental element in vector geographic information system.