Exemplary graphs. (a) An undirected, connected planar graph – the edge ae can be drawn to the left of vertex d, so that it will not intersect with the edge cd. Edges need not be straight lines. (b) A disconnected graph. Degree-zero vertices, such as g here, are allowed. (c) A directed graph. A path from, e.g., b to c is allowed, but to go back one needs to travel from c → a → b. (d) A mixed weighted graph. (e) The dashed edge is a bridge. After removing this edge, the graph becomes disconnected, so it yields λ(G) = 1.