Also known as the anti-theft lock. The entrance door is the gate of every family. It is a watershed outside the home. It must have a special function, which can play the role of insurance and security. The household usually consists of two doors: an iron gate and a general door (usually a wooden door). For the iron gate, both sides can be locked, commonly known as: double-headed lock. Commonly used are multiple safety door locks and mortise steel door locks. When selecting, it should be noted that the distance between the two doors should not be less than 80mm (8cm). Otherwise, the distance between the two doors (because of the handle of the lock) is not long enough to be closed at the same time and does not function as an anti-theft. The main door wooden door is generally equipped with a large handle lock, which can play an aesthetic role.