This is a question about the correct reference for a result that seems to appear frequently in the literature on planar graph isomorphism. In "A $V \log V$ Algorithm for Isomorphism of Triconnected Planar Graphs", Hopcraft and Tarjan mention that a 3-connected planar graph has an essentially unique embedding into the plane (up to mirror symmetry), and attribute this result to Whitney (1933) "A set of topological invariants for graphs". I've noticed this citation reappearing in various places (for example here, here, and here), but I couldn't find the result in Whitney's article itself -- am I just overlooking something? The paper is dense, and builds on a series of other articles he published in succession... Still, this result sounds a lot like Theorem 11 of Whitney (1932) "Congruent Graphs and the Connectivity of Graphs", which states that a 3-connected planar graph has a unique dual. Was that the intended reference?