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A real-estate lock box is a padlock-shaped box that hangs around the doorknob of a house that is on the market. The device holds the keys to a house to allow communal access for all real estate agents, while continuing to keep them secure. Permission is generally required from the seller, who is the owner, to install such a device on the door.
Real estate lock boxes are secured either with a manual key, a security code, or a swipe card. High-tech versions of this device allow homeowners to know when the home has been accessed.
Video Real-estate lock box
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Source of the article : Wikipedia