A stair, or a stairstep is one step in a flight of stairs.[1] In buildings, stairs is a term applied to a complete flight of steps between two floors. A stair flight is a run of stairs or steps between landings. A staircase or stairway is one or more flights of stairs leading from one floor to another, and includes landings, newel posts, handrails, balustrades and additional parts. A stairwell is a compartment extending vertically through a building in which stairs are placed. A stair hall is the stairs, landings, hallways, or other portions of the public hall through which it is necessary to pass when going from the entrance floor to the other floors of a building. Box stairs are stairs built between walls, usually with no support except the wall strings.[1]Stairs may be in a straight run, leading from one floor to another without a turn or change in direction. Stairs may change direction, commonly by two straight flights connected at a 90 degree angle landing. Stairs may also return onto themselves with 180 degree angle landings at each end of straight flights forming a vertical stairway commonly used in multistory and highrise buildings. Many variations of geometrical stairs may be formed of circular, elliptical and irregular constructions.[1]Stairs may be a required component of egress from structures and buildings. Stairs are also provided for convenience to access floors, roofs, levels and walking surfaces not accessible by other means. Stairs may also be a fanciful physical construct such as the stairs that go nowhere located at the Winchester Mystery House. Stairs are also a subject used in art to represent real or imaginary places built around impossible objects using geometric distortion, as in the work of artist M. C. Escher."Stairway" is also a common metaphor for achievement or loss of a position in the society; or as a metaphor of hierarchy (e.g. Jacob's Ladder, The Battleship Potemkin).StepThe step is composed of the tread and riser.TreadThe part of the stairway that is stepped on. It is constructed to the same specifications (thickness) as any other flooring. The tread "depth" is measured from the outer edge of the step to the vertical "riser" between steps. The "width" is measured from one side to the other.RiserThe vertical portion between each tread on the stair. This may be missing for an "open" stair effect.NosingAn edge part of the tread that protrudes over the riser beneath. If it is present, this means that, measured horizontally, the total "run" length of the stairs is not simply the sum of the tread lengths, as the treads actually overlap each other slightly.Starting step or BullnoseWhere stairs are open on one or both sides, the first step above the lower floor may be wider than the other steps and rounded. The balusters typically form a semicircle around the circumference of the rounded portion and the handrail has a horizontal spiral called a "volute" that supports the top of the balusters. Besides the cosmetic appeal, starting steps allow the balusters to form a wider, more stable base for the end of the handrail. Handrails that simply end at a post at the foot of the stairs can be less sturdy, even with a thick post. A double bullnose can be used when both sides of the stairs are open.Stringer, Stringer board or sometimes just StringThe structural member that supports the treads and risers in standard staircases. There are typically three stringers, one on either side and one evenly spaced in the center, with more added as necessary for wider spans. Side stringers are sometimes dadoed to receive risers and treads for increased support. Stringers on open-sided stairs are called "cut" stringers.WindersWinders are steps that are narrower on one side than the other. They are used to change the direction of the stairs without landings. A series of winders form a circular or spiral stairway. When three steps are used to turn a 90° corner, the middle step is called a kite winder as a kite-shaped quadrilateral.TrimVarious forms of moldings are used to decorate and in some instances support stairway elements. Scotia or quarter-round are typically placed beneath the nosing where a riser and tread to support its overhang.