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Some of the old movies I have watched show “the boss” with an entrance leading to his/her office with large wording on the door — private. That means only the boss can go into his/her private place. Perhaps more particularly to the average person in that office, it means you may not open that door without a specific invitation from “the boss” to enter the private area.
I also observed some movies have young children with signs like that on their doors. They are at the point in life ...