Glossary/Overcoil
Overcoil
An overcoil refers to a modification of the hairspring, in which the outer coil of the spring is bent up and and then parallel over the inner coils of the spring. An overcoil allows the hairspring to store and release energy more evenly around its center than a flat hairspring, by flexing more concentrically.
There are many varieties of overcoils, the aim of all of which are to help to eliminate errors in the rate of timekeeping of a timepiece that are due to changes in its position.