Sophus Lie and Felix Klein first met in 1869 as students in Berlin. They soon became daily companions and spent the spring of 1870 together in Paris where they met the French mathematicians Michel Chasles, Gaston Darboux, and Camille Jordan. Jordan had just published his classic Traité des substitutions, and the two foreigners read it avidly. Mathematics has not been the same since, for it has often been said – and not altogether unjustly – that from this moment on they made group theory their common property: Lie taking the continuous groups and Klein those that were discontinuous. It should not be overlooked, on the other hand, that this observation was first made by Klein himself in the preface to his Lectures on the Icosahedron (Klein 1884, iv), making this an effective piece of propaganda for a particular view of their early work in geometry.