Some graphs are in two or more parts. For example, consider the graph whose vertices
are the stations of the London Underground and the New York Subway, and
whose edges are the lines joining them. It is impossible to travel from Trafalgar Square
to Grand Central Station using only edges of this graph, but if we confine our attention
to the London Underground only, then we can travel from any station to any other. A
graph that is in one piece, so that any two vertices are connected by a path, is a connected
graph; a graph in more than one piece is a disconnected graph (see Fig. 1.9).
We discuss connectedness in Chapter 3.