Affine deformations of a three-holed sphere

This paper was first written in 1990, but was never published. In it, the author presents a novel approach to the study of constant curvature spacetimes in 2+1 dimensions. A parameterization of flat 2+1-dimensional domains of dependence is given in terms of measured geodesic laminations. There is also an interesting reinterpretation of Thurston's Earthquake Theorem involving anti-de Sitter spacetimes. With the permission of the author, it will be published for the first time in a forthcoming issue of Geometriae Dedicata, together with detailed "notes" outlining the developments in the field in the intervening years. The version posted here is nearly identical to the original; we merely corrected typographical errors and occasional notational mistakes, and also updated the references in the bibliography.