We discuss all possible compactifications on flat three-dimensional smooth spaces. In particular, various fields are studied on a box with opposite sides identified, after two of them are rotated by $π$, and their spectra are obtained. The compactification of a general 7D supersymmetric theory in such a box is considered and the corresponding four-dimensional theory is studied, in relation to the boundary conditions chosen. The resulting spectrum, according to the allowed field boundary conditions, corresponds to partially or completely broken supersymmmetry. We briefly discuss also the breaking of gauge symmetries under the proposed box compactification.

11 pages, 1figure. As it appeared in Phys. Lett. B