Door to door security door salesmen that sold security door chains to duped door owners must have laughed at the ease with which they traded this non-safe implement. Simple to install and operate the door chain was not a very good deterrent against unwelcome callers. The useless door chain was marketed as an extra lock yet the door’s main lock had to be unlocked leaving the chain as the only defence against outsiders.
It would not take much force for a burglar, stalker or restraining order abuser to open the door. A hefty shoulder charge would loosen the screws in the door frame and access would be easy for the intruder. A safer alternative is the spy hole. The householder can see who is outside their door.
Unless the caller puts his thumb against the spy glass in which case after a -Who‘s there? Guess who? – exchange you think you recognise the voice and accordingly, open the door or keep the door closed.
Boadicea's Chariot 