A barrel in watchmaking is a cylindrical component that houses the mainspring, which is the primary source of energy for a mechanical watch. The mainspring is a coiled strip of metal that, when wound, stores potential energy. As it unwinds, it releases this energy to power the movement of the watch. The barrel plays a crucial role in regulating the release of this energy, ensuring that it is distributed evenly over time to maintain accurate timekeeping.

The barrel is typically located in the movement of the watch and is connected to the gear train. It transfers energy to the gear train, which then drives the escapement and balance wheel, ultimately controlling the movement of the hands on the watch face. The design and construction of the barrel can significantly impact the performance and precision of a watch. Some watches feature a single barrel, while others may have multiple barrels to increase power reserve, allowing the watch to run for extended periods without needing to be wound.

In addition to its functional role, the barrel can also be a point of aesthetic interest in high-end watches, often being intricately decorated or visible through a transparent case back, showcasing the craftsmanship involved in its creation.