The paper by Bañados, Henneaux, Teitelboim, and Zanelli analyzes the geometry of spinning black holes in 2+1 dimensions with a negative cosmological constant and no matter couplings. The black hole is shown to arise from identifications of points in anti-de Sitter space by a discrete subgroup of the $SO(2,2)$ symmetry group. The metric is smooth everywhere except at $r=0$, where it is a singularity in the causal structure rather than the curvature. Coupling to matter introduces a curvature singularity at $r=0$. The paper discusses Kruskal coordinates and Penrose diagrams, focusing on the limiting cases of a massless, non-rotating hole and a maximally rotating hole. It also provides a classification of the elements of the Lie algebra of $SO(2,2)$ in an appendix.The paper by Bañados, Henneaux, Teitelboim, and Zanelli analyzes the geometry of spinning black holes in 2+1 dimensions with a negative cosmological constant and no matter couplings. The black hole is shown to arise from identifications of points in anti-de Sitter space by a discrete subgroup of the $SO(2,2)$ symmetry group. The metric is smooth everywhere except at $r=0$, where it is a singularity in the causal structure rather than the curvature. Coupling to matter introduces a curvature singularity at $r=0$. The paper discusses Kruskal coordinates and Penrose diagrams, focusing on the limiting cases of a massless, non-rotating hole and a maximally rotating hole. It also provides a classification of the elements of the Lie algebra of $SO(2,2)$ in an appendix.