# Fraysseix–Rosenstiehl planarity criterion

In the following figures, simple circle nodes represent vertices, double circle nodes represent subtrees. Twisted segments represent tree paths and curved arcs represent cotree edges (with label of the edge put near the curved arc). In the first figure, ${\displaystyle \alpha }$ and ${\displaystyle \beta }$ are T-alike (it means that their low extremities will be on the same side of the tree in every planar drawing); in the next two figures, they are T-opposite (it means that their low extremities will be on different sides of the tree in every planar drawing).