What does a mark of "X" on the door of a building signify?

A mark of "X" on the door of a building signifies that a search has been conducted. This marking is a part of the marking system used by firefighters and emergency responders to relay critical operational information to other members of the response team. When an "X" is placed on the door, it indicates that firefighters have already checked the structure, which helps ensure that valuable resources are not wasted by duplicating efforts.

In emergency situations, effective communication about the status of a building can save lives and streamline operations. The "X" communicates that a thorough search—for victims or hazards—has already taken place, allowing subsequent responders to focus on other critical aspects of the incident. This system is essential in maintaining organized and efficient responses in firefighting and rescue operations.