Properly designed and built, masonry arches can carry considerable loads. Why? Because under uniform loading, the individual masonry units that make up the arch are in compression, not tension. And masonry units--stone, clay, or concrete--are strongest in compression. Although there are several styles of arches, the semicircular and the segmental are the most stable and simplest to construct. A segmental arch has a circular curve, but less than a semicircle. Three structural conditions are essential to the integrity of an arch: the length of the span must remain constant; the elevation of the ends must remain unchanged; the inclination of the skewback must be fixed. Minor arches (those that span less than 6 feet and have a maximum rise-to-span ratio of 0.15) may carry loads from floors, roofs, walls, and other structural members. A common way to verify if a minor arch can support the required loads is the line-of-thrust method. In this method, the line of thrust is drawn from the skewback to the crown of the arch. If this curved line lies completely within the middle third of the arch section, the arch is stable. The steps required to draw the line of thrust for a masonry arch are illustrated.