 An orientation in which each face of a planar graph has an odd number of clockwise-oriented edges is automatically Pfaffian.Such an orientation can be found by starting with an arbitrary orientation of a spanning tree of the graph.The remaining edges, not in this tree, form a spanning tree of the dual graph, and their orientations can be chosen according to a bottom-up traversal of the dual spanning tree in order to ensure that each face of the original graph has an odd number of clockwise edges.