The Breguet Overcoil is a specialized hairspring design used in mechanical watch movements to enhance timekeeping precision. Named after the renowned watchmaker Abraham-Louis Breguet, this innovation involves an upward curve at the outermost coil of the hairspring. This design modification allows the spring to expand and contract concentrically, maintaining a consistent center of gravity and reducing positional errors that can affect the accuracy of a watch.

In traditional flat hairsprings, the coils can bunch together or expand unevenly, leading to variations in the rate at which the watch keeps time. The Breguet Overcoil addresses this issue by ensuring that the spring's coils remain more evenly spaced during operation. This results in improved isochronism, meaning the watch maintains consistent timekeeping regardless of the position in which it is worn.

The Breguet Overcoil is often found in high-end mechanical watches, where precision and reliability are paramount. Its implementation requires meticulous craftsmanship and expertise, as the overcoil must be carefully shaped and adjusted to function correctly. This feature is a testament to the watchmaker's skill and dedication to achieving superior performance in timekeeping. The Breguet Overcoil remains a hallmark of quality in horology, symbolizing the blend of tradition and innovation in watchmaking.