Progressive, super high quality, extra heavy duty, galvanized steel, safety crash chain door stop, with extension spring designed to absorb shock and preventing the door from swinging open and damaging itself. This door chain is unlike any other door chain available today. This is by far the strongest and heaviest. Designed for heavy exterior doors to withstand strong winds, like roof doors. (Please note, do NOT use this chain for screen doors or glass doors that dont have at least 15" solid door above the screen or glass. The chain is too bulky, too heavy, and its hanging down about 15", and it will damage the screen or the glass). The chain is covered with a heavy duty protective rubber (as seen in picture #2). To be installed on the push side of the door. Choose from 2 sizes. #RDC-36 for 34" and 36" doors. #RDC-30 for 30" and 32" doors. Designed to fit wood and steel doors up to 2-3/4" thick (It can fit any door thickness with longer bolts and nuts, but the bolts and nuts thats included is only for up to 2-3/4"). The 3" X 3" door mounting plate has to be installed on the door with the included bolts and nuts, bolted through the door. The 1-3/4" X 2-1/2" jamb (door frame) mounting plate has to be installed on the door frame. Mounting and fastening hardware includes only bolts and nuts for the door. We manufacture and include screws for wood door frames and self drilling screws for steel door frames. (see the self drilling screws in the pictures above). Easy to follow instructions. Please note, this door chain and spring does "NOT" slow the door down when opening or closing the door (a separate hydraulic door closer is required for that). The door will open and close as fast as the wind slams it. If the door has spring hinges, the door will open and close as fast as the spring hinges allows it. The only thing this chain and spring does, it stops the door from swinging open all the way. (See the specifications and mounting guide in the pictures above).