A layer can only be laid on the one below it, or across a gap between two supports. That gives the printable overhang, the bridge, and the reason support material is avoided: it costs material and post-processing and leaves the worst surface and the worst tolerance on the part [1]. The design answers are geometric. Horizontal edges get chamfers, since a fillet begins with an overhang the printer cannot make; a horizontal hole gets a pointed or flat roof, as the printed bearing seats already do after Slant 3D [2]; an upside-down counterbore gets a sacrificial layer or the stacked-bridge trick; and a part that no orientation can print without support is split or turned 45° on the bed. Bridges laid on bridges give geometry that looks impossible without support, as in the OpenFlexure microscope. The chapter ends with a test coupon of overhang angles, bridge lengths and hole roofs that the cohort prints to find the limits of our machines.