The swan neck, also known as the regulator spring, is a component on the top of a balance bridge that enables fine adjustment of the rate.
The swan neck was a feature of high-end watches until the second half of the 20th century. It was then largely replaced by the variable inertia balance-wheel, which no longer needs a index and therefore no swan neck.
The swan neck is a component that lends itself perfectly to the artisanal method. It can be angled at the top and bottom and can also have a black polish on its upper surface. On very high quality watches, the blade is cleared on the underside to prevent it from scratching the bridge.
The swan neck can sometimes be produced in a semi-artisanal way. For this method, it is produced by electro-erosion or stamping. The bevelling is summary.
Around 1960-1970, movements equipped with swan necks were produced on an industrial scale. This has since been largely abandoned in favour of variable inertia balance-wheels, which need neither a index nor a swan neck.