In the case of residential structures, the standard floor-to-floor height is 7.5 feet. In contrast, the height of each storey in a commercial structure is 10 feet. (see Fig. 2). The residential structure has four storeys, while the commercial structure has three. The difference in height between floors is called the stairway risers. On average, each riser is about 1 foot 6 inches high (for security reasons, the risers are usually not uniform in size). Stairs are used to cross from one level to another. They are also useful for access to upper rooms, such as garages and roof spaces. There are two types of stairs: straight-flight and curved-flight. Straight-flight stairs have steps that are either all the same length or vary in step height. These steps lead from one level to the next in a straightforward manner. Curved-flight stairs have steps that are of different lengths and/or heights; they lead the user through several levels before reaching the top. For example, a staircase with 30-inch risers has three flights of eight steps each. A flight is counted by starting at the first step on the lowest level and going up or down according to whether you are going upstairs or downstairs.