The nodes are laid out as dots on a sheet of paper, frequently labelled
by the members of the set N. The links are represented by arrows
joining the nodes, the arrow runs from node i to node j
if 
. Note that there is no requirement that both 
and 
belong to L, nor that 
either belongs or does not
belong.
It is a classical problem in graph theory, to decide whether a graph may be drawn in two dimensions so that its links do not cross; this point does not interest us except insofar as the aesthetics of laying out the graph is concerned.