The fire triangle or combustion triangle is a simple model, from the science of firefighting, for understanding the ingredients necessary for most fires. It has largely been replaced in the industry by the fire tetrahedron, which provides a more complete model, also described below.
The triangle illustrates the rule that in order to ignite and burn, a fire requires three elements: heat, fuel, and an oxidizing agent, usually oxygen. The fire is prevented or extinguished by removing any one of them. A fire naturally occurs when the elements are combined in the right mixture (e.g., more heat is needed for igniting some fuels, unless there is concentrated oxygen).
Without fuel, a fire will stop. Fuel can be removed naturally, as where the fire has consumed all the burnable fuel, or manually, by mechanically or chemically removing the fuel from the fire. Fuel separation is an important factor in wildland fire suppression, and is the basis for most major tactics. Other fuels may also be chemically altered to prevent them from burning at ordinary temperatures, perhaps as part of a fire-prevention measure.
Without sufficient heat, a fire cannot begin, and it cannot continue. Heat can be removed by dousing some types of fire with water; the water turns to steam, taking the heat with it. Note that water will actually increase or spread some other types of fires (such as combustible metal fires, see comments below). Separating burning fuels from each other can also be an effective way to reduce the heat. In forest fires, burning logs are separated and placed into safe areas where there is no other fuel. Scraping embers from a burning structure also removes the heat source. Turning off the electricity in an electrical fire removes the heat source, although other fuels may have caught fire and continue burning until the firefighter addresses them and their fire triangles too.
Without sufficient Oxygen, a fire cannot begin, and it cannot continue. Oxygen may be removed from a fire by smothering it with an aqueous foam, or some inert gas (e.g., carbon dioxide) or dry chemicals, or by enclosing it where the fire will quickly use up all of the available oxygen. A candle snuffer uses this principle. Oxygen for the fire may also be instantantly consumed, if only for a moment, by more sophisticated means such as using explosives to 'snuff' an oil well gas fire. Once the gas fire is out, it is not hot enough to start again, but workers must be extremely careful not to create sparks.
The fire triangle is a useful teaching tool, but fails to identify the fourth essential element of fire: the sustaining chemical reaction.
This has led to development of the fire tetrahedron: a triangular pyramid having four sides (including the bottom). Some fire suppression agents do not remove or reduce any of the three necessary components, but rather interfere with their chemical combination, such as Halon. In most fires, it does not matter which element gets removed; the fire fails to ignite, or it goes out. However, there are certain chemical fires where knowing only the "fire triangle" is not good enough.
Combustion is the chemical reaction that feeds a fire more heat and allows it to continue. With most types of fires, the old fire triangle model works well enough, but when the fire involves burning metals (known as a class-D fire in the American system of fire classifications, involving metals like lithium, magnesium, etc.), it becomes useful to consider the chemistry of combustion. Putting water on such a fire could result in the fire getting hotter (or even exploding) because such metals can react with water in an exothermic reaction to produce flammable hydrogen gas. Therefore, other specialized chemicals must typically be used to break the chain reaction of metallic combustion and stop the fire.