2.5.4. Labelled Graphs
The vertices or the edges of a graph may be labelled with numbers or other symbols.
Although the vertices of a graph are normally labelled with domain values, it is possible to add further labels to them, representing the values of one or more functions of the vertices. For example, a graph of airline connections might show the time zone of each airport.
When the edges are labelled, the graph then represents a ternary relation. In a labelled graph, it isn’t unusual to have more than one edge from vertex u to vertex v, provided they have different labels. For example, a labelled graph may contain both the edges 
and
and Get Systems Analysis and Synthesis now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.
