Interior Elevations differ from Sections in that Interior Elevations begin at the finished floor elevation and end at the ceiling. Interior elevations depict individual walls in distinct rooms, whereas floor sections depict many rooms piled on top of each other. For example, a house with two floors and four units per floor would have eight interior walls and sixteen floor surfaces. Sections are used by architects and engineers to show the overall layout of a building or structure. The section may be taken at any point within the building where there is a change in direction of a wall or column. These changes can occur when a room opens up to another space or when an exterior wall is added to a building. The section process begins with measuring and marking all internal angles and intersections of the proposed wall arrangement. From these points, lines are extended in several directions to create several section planes. On large-scale drawings, it is common to see several section views drawn to different scales so that one can visualize the whole building at one glance. It is important to note that sections are not precise replicas of actual buildings. They are simply illustrations used by architects to show the overall shape of a structure while also providing detail about its construction. In addition, sections are designed to hide physical obstructions such as plumbing and electrical wiring, so they do not show actual holes for pipes and cables.