We finally come to the question of why the black hole horizon is said to allow only one-way traffic. When viewed from the Kruskal coordinates, suitable for freely falling observers, the horizon consists of several distinct causal components. The future event horizon is the one we usually refer to when describing the one-way nature of the black hole geometry; its “past” cousin allows the opposite flow of trajectories but is often an artifact of the “eternal” geometry. We derive and display Penrose diagrams for many of the solutions accumulated so far and offer cautionary tales on causal structures and singularities.