Cog: Gear tooth versus wooden joint

To most people, a cog is one of the teeth on a gear wheel. It catches and meshes with teeth on another gear to transmit rotational force. Cogs are the moving parts of clocks, mills, and machinery; their name carries overtones of precision, mechanical purpose, and industrial work.

In carpentry, a cog is something entirely different. It is a wooden projection or tenon cut at the end of one beam that slots into a matching rectangular opening (the mortise) cut into another beam. The cog joint is a traditional form of timber framing used especially in roof construction and in heavy timber structures. The projection can be simple or stepped, and its depth and fit determine the strength of the connection. This joint transfers both tension and shear loads between members.

The two meanings descend from a single root. The mechanical gear tooth took its name from the wooden joint, not the other way around. As mills and machinery became central to industrial production, cog became the dominant term for the metal or cast iron teeth on wheels. The wooden joint kept the older name because it too resembles a protruding tooth that engages with a matching socket.

The divergence reflects the material history of technology. Wooden gears with cog-like teeth appeared in mills and water wheels well before precision metalworking made steel gears commonplace. The language of carpentry and the language of mechanical engineering both reached for the same metaphor: a tooth-like projection that locks things together. Over centuries, one path led into the foundry and the machine shop; the other remained in the timber frame.