2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/54916The parameter-space orbifold construction of open and unoriented toroidal and (target-space) orbifold compactifications is briefly reviewed, with emphasis on the underlying geometrical framework. A class of chiral four-dimensional type-I vacua with three generations is also discussed.12 pages, latex, no figures; contribution to the Proceedings of the 12th Italian Conference on General Relativity and Gravitational Physics, Rome, september 23-27, 1996High Energy Physics - Theorytext