One of the simplest forms of stairways in use is a step ladder, fixed or movable, such as is sometimes built from an attic floor to the roof, as is shown in Fig. 18, where (a) is a plan, and (b) an elevation. The top step should always have an increased width, and the sides of the ladder should be at least 6 inches wide by 1 1/8 inches thick. The steps are gained into the sides not less than 3/8 inch and securely nailed or screwed thereto. This ladder rises 8 feet, or 96 inches, which, divided by 10, the number of steps, gives 9| inches for each riser. The run, or horizontal distance from the face of the beam at the landing to the bottom of the ladder, equals 3 feet 7 inches, less 3 inches for additional width of top step, leaving 3 feet 4 inches, or 40 inches, which, divided by 10, the number of treads, gives 4 inches as the width of each tread from a vertical line dropped from the nosing above.