Introduced by J. A. Lepine in about 1770. This layout made possible a much thinner watch, particularly with countersinks in the dial plate and a hanging going barrel. Basically, an escapement in which impulse is imparted to the balance by means of a lever, having at one end a pair of pallets which engage with the teeth of the escape wheel, and at the other a fork with a notch, into which the impulse pin on a roller fixed to the balance staff enters, receives impulses from the lever, and exits. In the former the lift is on the pallets, and in the latter it is divided between pallets and teeth. The action of both is fundamentally the same.
V shows the right-angled English form with single or table roller.
In the last quarter of the 19th century the double roller was used on high quality watches, the safety action provided by the dart taking place on a separate and smaller roller located below the impulse roller. The Swiss form has two rollers as seen in Fig. Here, A designates the escape wheel. B and C are Fig. D is the dart or guard pin.