 This issue was tackled in the 1840s and 1850s by Georg Karl Christian von Staudt. His two books (1847, 1856–1860) attempted to give foundations for projective geometry that made it an autonomous subject, independent of Euclidean geometry. They were hard to read, and imperfect in a number of ways, but the task of creating a rigorous theory could be seen for the first time as a matter of completing a task already begun. Von Staudt argued that the transformations of plane projective geometry could map any triple of collinear points to any other, and any quadruple of points (no three of which were collinear) to any other, but not any quadruple of collinear points to any other. He then made a detailed study of collinear quadruples. He also made brief remarks about how Euclidean geometry could be obtained from projective geometry, and from these it could be seen that his theory of collinear quadruples reduced to the familiar theory of cross-ratio as soon as the concept of Euclidean distance was added to projective geometry. This insight was made clear and explicit by Felix Klein in a number of papers in the early 1870s. The first readable textbook on projective geometry, and the one that gave it its name, was Cremona’s Elementi di geometria projettiva of 1873, and after that the subject swiftly rose to become the fundamental classical geometry.