<div class="htmlview paragraph">The free, resilient, self-expanding, one-piece piston-ring is a product of strictly modern times. It belongs to the internal-combustion engine principally, although it is applicable to steam engines, air-compressors and pumps. Its present high state of perfection has been made possible only by the first-class material now available and the use of machine tools of precision.</div> <div class="htmlview paragraph">The author outlines the history of the gradual evolution of the modern piston-ring from the former piston-packing, giving illustrations, shows and comments upon the early types of steam pistons and then discusses piston-ring design. Piston-ring friction, the difficulties of producing rings that fit the cylinder perfectly and the shape of rings necessary to obtain approximately uniform radial pressure against the cylinder wall are considered at some length and illustrated by diagrams. The best material for piston-rings is stated to be cast iron; the average of 10 different analyses of satisfactory cast iron used for piston-rings is given; the piston-ring specification of one of the large motor-car manufacturers is presented; piston-ring joints and future design are considered; and a recommendation is made toward piston-ring standardization.</div>