Beltrami–Klein model, also called the projective model, Klein disk model, and the Cayley–Klein model, is a model of n-dimensional hyperbolic geometry in which points are represented by the points in the interior of the n-dimensional unit ball (or unit disk, in two dimensions) and lines are represented by the chords, straight line segments with endpoints on the boundary sphere. It made its first appearance in two memoirs of Eugenio Beltrami published in 1868, first for n=2 and then for general n, devoted to showing equiconsistency of hyperbolic geometry with ordinary Euclidean geometry