The fusee and chain mechanism is a historical horological device used to regulate the power delivery from the mainspring to the gear train in mechanical watches and clocks. This system was developed to address the problem of uneven force exerted by the mainspring as it unwinds. The fusee is a conical, spiral-shaped pulley connected to the mainspring via a chain. As the mainspring unwinds, the chain wraps around the fusee, which compensates for the decreasing force of the mainspring by increasing the leverage. This ensures a more consistent torque is delivered to the gear train, improving the timekeeping accuracy of the watch or clock.

The chain, typically made of metal, is a critical component that links the mainspring barrel to the fusee. It is meticulously crafted to withstand tension and ensure smooth operation. The use of the fusee and chain mechanism was prevalent in the 16th to 19th centuries, particularly in high-quality timepieces. Although modern watchmaking has largely moved away from this system in favor of more compact and efficient solutions, the fusee and chain remain a symbol of traditional craftsmanship and mechanical ingenuity, often found in vintage and collectible timepieces.