Poly3d: a three-dimensional, polygonal element, displacement discontinuity boundary element computer program with applications to fractures, faults, and cavities in the earth’s crust. Poly3D is a C language computer program written by Andy Thomas (1993) to calculate the quasi-static displacement, strain and stress fields in a linear-elastic, homogeneous and isotropic whole- or half-space using planar, polygonal elements of displacement discontinuity and the boundary element method (BEM). The elements may have any number of sides, with a minimum of three, and the solution by Comninou and Dunders (1975) for an angular dislocation is used for their construction. Geologically, a polygonal element may represent some portion, or all, of a fracture or fault surface (see figure below). The displacement discontinuity (fracture aperture or fault slip) is constant on each element, but one may use multiple elements to model an arbitrary number of mechanically-interacting fractures or faults with non-uniform opening and/or slip distributions.