When masonry is aid in a running bond pattern, it creates a natural corbelled arch which transfers much of the vertical load to either side of the opening. The area inside a triangle with sides at 45 degrees angles to the lintel represents the masonry which must be supported by the lintel. Outside this area, the weight of the masonry is assumed to be carried to the supporting abutments by natural arching. For this assumption to be true, however, the arching action must be stabilized by a minimum of 200 mm to 400 mm of masonry above the top of the triangle. There must also be sufficient masonry mass on both sides of the opening to resist the horizontal thrust, and there cannot be a movement joint at either side of the opening. If arching action cannot be assumed to occur because of inadequate height above the load triangle, inadequate thrust resistance, movement joints locations, or because the masonry is not laid in running bond, the lintel must be sized to carry the full weight of the masonry above its entire length. When arching action is assumed, the lintel requires temporary support until the mortar has cured sufficiently to allow the masonry to assume its share of the load.