What a desktop printer does, and what that does to the size of a part. The nozzle lays a road of plastic about one nozzle diameter wide, layer on layer, and the reference profile of this book is the one most printers share: a 0.4 mm nozzle at 0.2 mm layer height [1]. From it follow the smallest feature that exists (two extrusion widths for a wall or a gear tip, as in the printed pinion), the deviation of about ±0.1 mm per surface that the GT2 pulley runs into, holes that come out undersize and bosses oversize, elephant foot on the first layers, and the seam where each perimeter closes, which turns a round hole into one that is neither round nor centred. The designer’s answers are geometric: fillets on vertical edges so the head never stops in a corner, a teardrop or a notch that gives the seam somewhere to go, and offsets tuned by test prints when nothing else will do, as in the printed bearing, valid for one printer only. The ISO/ASTM 52900 terms and the 52921 orientation notation are fixed here and used through the part. Our own number replaces the rule of thumb: a dimensional test coupon printed on all 54 printers, giving the scatter of hole and boss sizes the other chapters design for.