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 personal 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.
Where there are teams working inside multiple entrances of a large structure fire, there may be standby teams designated at each entry point, although the rule does not necessarily mean two firefighters ready outside for every two inside.
Note that these rules also apply to all training with live fires inside structures, and are even more important when inexperienced trainees are inside.
The two firefighters who are specifically designated as standing by outside are referred to in different ways by many localities. Some are referred to as a rapid entry team (R.E.T.), or "rapid intervention team", while others are referred to as a F.A.S.T. Truck (as in the Fire Department of New York), meaning "firefighter assist and search team".
Other tactics work within the rule to "stage" the next team, ready to relieve the inside team when their air supplies run low. When the third team arrives to serve as the RET, the second team enters, follows the hose line to the first team and relieves them. This is naturally smoother with good radio communications between officers.
Until 1999, the firefighting in France was performed according the "Rules of instruction and maneuvers" (Règlement d'instructions et de manoeuvres, RIM). The firefighters acted in teams of three members, called "trinomial" (trinôme): one chief, one deputy chief and one support; a fire engine with eight people thus had two trinom, an engine chief (chef d'agrès) and a driver. In the 1990s, the Paris Fire Brigade started to work with teams of two people, called "binomial teams" (équipes en binôme) or shortly "binomials" (binômes). This organisation was generalised to the whole France in a departmental order signed by the minister of Internal Affairs the 3 February 1999. An engine with eight people now has three binomials, an engine chief and a driver; an engine with six people has two binomials, an engine chief and a driver.
A binomial can be assigned two types of missions:
The mission can change during an intervention, i.e. an attack binomial can become a supply binomial or vice versa.
The "usual" intervention (simple fire with only one fire engine and six people, no casualty, no specific risk) thus involves two binomials.
The overall organisation is the same when more engines are involved.
Last Men Out (ISBN 0-8050-7169-5) A book about Rescue Team 2 of the NYFD, whose job is to rescue other firefighters from burning buildings.