  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.
