This free tattoo design is a door knocker, which is an item of door furniture that allows people outside a house to alert those inside to their presence. A door knocker has a part fixed to the door, and a part (usually metal) attached to it by a hinge that may be lifted and used to strike a plate fitted to the door, or the door itself, making a noise. The struck plate, if present, would be supplied and fitted with the knocker. Such as this one.