An emergency exit (also known as a fire exit) is a special exit used during emergencies in a building or other structure such as fires. The combined use of regular and emergency exits allows for faster evacuation, and emergency exits provide alternative means of evacuation if regular exits are inaccessible.

Emergency exits must:

An emergency exit's path usually ends in an outward-opening door with a crash bar with exit signs pointing to it. It is usually a door to an area outside of the building, but may also lead to an adjoining, fire-isolated structure with clear exits of its own.

A fire escape is a special kind of emergency exit consisting of stairs and/or extendable escape ladders mounted on the outside of a building.

Local building codes or building regulations often dictate the number of fire exits required for a building of a given size, including the number of stairwells. For any buildings bigger than a private house, modern codes invariably specify at least two sets of stairs, completely isolated from each other so that if one becomes impassable due to smoke or flames, the other remains usable.

The traditional way to satisfy this requirement was to construct two separate stairwell stacks, each occupying its own footprint within each floorplan. Each stairwell is internally configured into an arrangement often called a "U-return" or "return" design. The two stairwells may be constructed next to each other, separated by a fireproof partition, or optionally the two stairwells may be located at some distance from each other within the floorplan. The traditional arrangement has the advantage of being easily understood by building occupants and occasional visitors.

Some architects save space while still meeting the exit requirement, by housing two stairwells in a "double helix" or "scissors stairs" configuration whereby two stairwells occupy the same floor footprint, but are intertwined while being separated by fireproof partitions along their entire run. However, this design deposits anybody descending the stack into alternating locations on each successive floor, and this can be very disorienting. Some building codes recommend using a color-coded stripe and signage to distinguish otherwise identical-looking stairwells from each other, and to make following a quick exit path easier.

In older buildings that predate modern fire codes, and which lack space for a second stairwell, having intertwining stairs so close to each other may not allow firefighters going up and evacuees going down to use separate stairways.