While most action parts are made of hard maple, the Hammer Molding itself is sometimes made from mahogany or a similar hardwood. It is attached to a Hammer Shank which may be cylindrical or octagonal as shown here. Underneath the hammer shank is attached the Knuckle.
It is this Knuckle that is actually pushed up by the jack (shown later). When the jack slips from underneath the Knuckle, the hammer is said to "escape" as it surges upward suddenly and strikes the string.
The Hammer Felt, the covering over the hammer, must be of exact hardness and shape to produce the best possible tone when striking the string.