The chain clasp is installed on the back of the door, not far from the door. One end of the slide chain is fixed on the door frame, and the other end is active. It can be locked in the chain clasp on the back of the door. When the anti-theft chain is hung, the door can only open up to about 5 to 8 centimeters. People cannot pass it. The hand cannot reach through the door to reach the cufflinks on the back of the door. This ensures that the owner confirms the visit at the door. When you are a person, you will not be attacked by the other person. Since the slide chain is easily cut by a metal cutting forceps or the like, the anti-theft door chain is rarely used alone as a door lock, but is used as an anti-theft tool. Therefore, the slide chain type anti-theft door chain is also known as the "heart lock", which is intended to remind people to be extra careful, just in case. There are also "safety buckles" that use metal rods instead of metal chains, and they are designed with the same considerations.