Women are not supposed to show their hair to male strangers. It is thus compulsory for women to wear a veil in public spaces. But at home, when they are with their family members, they can free themselves.
But what if someone knocks at the door? Well, if the guest is a woman, then there is no need to cover as the prohibition applies only to male strangers. But if it’s a man, then women need to cover themselves.
But how to know if who knocks at the door is a man or a woman?
Well, at the door two doorknockers that produce different sounds are applied. Men use that that produces a lower sound, women that that produces a higher sound.
Brilliant, isn’t it?