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A firebox is a built-in unit that consists of a fire chamber surrounded by aing material, typically masonry. The firebox is the area of the fireplace where the fuel is burned and it must be constructed of non-combustible materials. The most common type of fuel used in fireplaces is wood, but other options such as natural gas, propane, or electricity can be used as well. The firebox must be large enough to accommodate the chosen fuel, but it should not be so large that it significantly diminishes the efficiency of the fireplace. In addition, the firebox must be properly ventilated to ensure that gases and smoke are properly directed up the chimney. A well-designed firebox is essential to the function and safety of any fireplace.