Metrical geometry is a part of descriptive geometry 1 , and descriptive geometry is all geometry. Arthur Cayley On October 5-th 2001, the authors of this book typed in the word "Schwarzian" in the MathSciNet database and the system returned 666 hits. Every working mathematician has encountered the Schwarzian derivative at some point of his education and, most likely, tried to forget this rather scary expression right away. One of the goals of this book is to convince the reader that the Schwarzian derivative is neither complicated nor exotic, in fact, this is a beautiful and natural geometrical object. The Schwarzian derivative was discovered by Lagrange: "According to a communication for which I am indebted to Herr Schwarz, this expression occurs in Lagrange's researches on conformable representation 'Sur la construction des cartes géographiques' " [117]; the Schwarzian also appeared in a paper by Kummer in 1836, and it was named after Schwarz by Cayley. The main two sources of current publications involving this notion are classical complex analysis and one-dimensional dynamics. In modern mathematical physics, the Schwarzian derivative is mostly associated with conformal field theory. It also remains a source of inspiration for geometers. The Schwarzian derivative is the simplest projective differential invariant, namely, an invariant of a real projective line diffeomorphism under the natural SL(2, R)-action on RP 1. The unavoidable complexity of the formula for the Schwarzian is due to the fact that SL(2, R) is so large a group (three-dimensional symmetry group of a one-dimensional space). Projective geometry is simpler than affine or Euclidean ones: in projective geometry, there are no parallel lines or right angles, and all nondegenerate conics are equivalent. This shortage of projective invariants is This curve is non-degenerate at point γ(0). The plane, dual to point γ(t), is given, in an appropriate affine coordinate system (a 1 , a 2 , a 3) in RP 3 * , by 1.2. DISCRETE INVARIANTS AND CONFIGURATIONS 1.5. SCHWARZIAN DERIVATIVE AS A COCYCLE OF DIFF(RP 1) 19 counterpart of the Huygens construction of the involute of a plane curve using a non-stretchable string: the role of the tangent line is played by the osculating conic and the role of the Euclidean length by the projective one. Affine differential geometry and the corresponding differential invariants appeared later than the projective ones. A systematic theory was developed between 1910 and 1930, mostly by Blaschke's school. 1.5 Schwarzian derivative as a cocycle of Diff(RP 1) The oldest differential invariant of projective geometry, the Schwarzian derivative, remains the most interesting one. In this section we switch gears and discuss the relation of the Schwarzian derivative with cohomology of the group Diff(RP 1). This contemporary viewpoint leads to promising applications that will be discussed later in the book. To better understand the material of this and the next section, the reader is recommended to consult Section 8.4. Invariant and relative ...