Wheel made from a circular toothed disc and a cylindrical box closed by a cover. The barrel turns freely on its arbor and contains the mainspring which is hooked to the barrel at its outer tip and to the arbor at its inner tip. The barrel engages with the first pinion of the gear train. As it slowly rotates, its angle of rotation varies between one-ninth and one-sixth of a revolution per hour. A hanging barrel is attached at its upper end only to the lower bottom plate or to the bridge. A plain barrel has no teeth and is used in fusee watches, with a chain or catgut coiled round the plain barrel and connecting it to the fusee.