In firefighting, the policy of two-in, two-out mandates that firefighters never go into a dangerous situation in a fire or rescue incident alone. There should always be (at least) two firefighters together when they enter a location and one of them cannot come out of the situation or building unless both do.
The policy also refers to a safety system to protect firefighters, where two or more firefighters enter a building and at least two more remain outside, ready to help in case of emergency. Firefighters will enter a building in teams to extinguish the fire and/or make a rescue. When a team enters an IDLH atmosphere (Immediately Dangerous to Life or Health) (the "two in"), two more firefighters (the "two out") will be standing by at the entrance in full personnal protective equipment (to include bunker gear and SCBA), and ready with rescue tools, in order to rapidly enter the building if the team inside becomes endangered. By some interpretations, the rule requires at least two more firefighters to remain outside, even when the standby team has gone in to find and rescue the first team. However, the rule specifically exempts an emergency rescue with fewer personnel on hand (i.e., no additional "two-out" required if they go in to find the "two-in"), although the increased risk to all four should be obvious if further backup is not on-scene.