This numeric lock box hangs easily on any door knob, fence or railing to store a conventional door key where it will be available to whomever you give the combination. Since it's a 4-digit lock box, you'll get more combination code choices than a standard 3-digit home lock box. No tools are required for installation; just open the key box, lift the shackle and hang over your door knob. The scratch-resistant shackle includes a rubber bumper that protects the door surface. The black, 4-digit lock box has a numeric combination preset to 0-0-0-0, but you can set your own combination code, and change it, at any time.