Page "Outerplanar graph" Paragraph 12

Every outerplanar graph is also a subgraph of a series-parallel graph.

However, not all planar graphs and series-parallel graphs are outerplanar: the complete graph K < sub > 4 </ sub > is planar but neither series-parallel nor outerplanar, and the complete bipartite graph K < sub > 2, 3 </ sub > is planar and series-parallel but not outerplanar.

Every forest, and every cactus graph is outerplanar.