Mother and child will have the function of preventing the card from being inserted into the lock tongue, but the normal state of the lock tongue not locking the single lock should be uneven strength. See you use it. It's better to use mother and son's for ordinary entrance doors. It's much safer when you forget the lock. But remember to bring the key. Under normal circumstances, the closing force of mother and son is smaller than ordinary ones, and it is easier to be turned off by the wind. (My understanding is that the lock is divided into three sections by the oblique tongue, and one section is the small tongue in the middle of the pin shaft that can swing.)