A clapper is an oblong piece of wood with one end narrower than the other and usually with a groove in each side. The groove allows to hold the clapper comfortably. The top and bottom are flat.
A clapper is used to flatten areas that are already pressed but may remain resilient and need further flattening. The area is dampened with a cloth and later pressed with the clapper exerting pressure for a few seconds. As clapper absorbs moisture it is turned over constantly to use the cool side. It is cautiously used to avoid marking the garment.