pene

Middle English pene meant the head of a hammer, specifically the end opposite the flat striking face. It likely came from Old French pene or panne, meaning the side or flap of something, possibly related to Latin penna, "feather" or "fin." The peen was the hammer's other face—the specialized end designed for a specific task rather than general striking.

Different peen shapes do different jobs. A ball-peen hammer has a rounded hemispherical peen, used for peening rivets (mushrooming their ends to lock them in place), shaping sheet metal, and striking punches and chisels. A cross-peen has a wedge-shaped edge perpendicular to the handle, used for starting nails and spreading metal. A straight-peen has the wedge parallel to the handle. Each shape is optimized for a specific metalworking operation.

Shot peening—bombarding a metal surface with small steel balls—became an industrial process in the 1920s. The repeated impacts compress the surface layer of the metal, creating residual compressive stress that dramatically increases fatigue resistance. Aircraft landing gear, automotive springs, and turbine blades are all shot-peened. The ancient blacksmith's technique of hammering metal to strengthen it was scaled up to industrial precision.

The ball-peen hammer is so fundamental to metalwork that it appears in logos, union insignia, and tool company trademarks worldwide. It is probably the most drawn hammer in history—more recognizable than a claw hammer to anyone who works with metal. The peen—the part of the hammer that isn't the obvious part—is what makes a metalworking hammer a metalworking hammer.