A printed part is weakest where layers are pulled apart, so the first design decision is the orientation on the bed, and the rest of the design follows from it [1]. The mechanics of the anisotropy is in Material properties and the tensile test: roads within a layer are continuous, layers are bonded only where they fused. This chapter turns it into rules with numbers. Tension and bending stresses go in the plane of the layers; a hook, a clip or a gear tooth is drawn in that orientation from the start; and when no orientation suits every load, the part is split and joined, a dovetail or a screw letting each piece be printed its own way. The practitioner figure is about three times the strength in the best orientation against the worst, and the academic reviews agree on the direction but not on one number [2,3]. Our number comes from the cohort tensile lab, where groups print the same specimen flat, on edge and standing against their own reference, with scatter and a lower bound. The same rule decides fatigue (layer boundaries act as notches) and why the yield criteria for isotropic materials do not apply as they stand. An orthotropic model calibrated on the same data is the step from a rule to a design tool.