This system owes its name to the fact that, normally, the vertical elements (uprights) are secured to the load- bearing structure first. Once the uprights are installed, the horizontal elements (crossbars) are added and lastly, the glazed panels. See-through glazed panels are composed of insulated glass walls while blind panels can be comprised of monolithic glass and rear panels or metal elements, e.g.: steel, aluminium, titanium, or composite materials such as Alucobond. Both the horizontal and vertical elements that make up the frame are generally composed of aluminium extrusion profiles. The proper installation of the seals ensures optimal air and water tightness. The expansion of the elements are absorbed and compensated among the various elements themselves. The glass panes and panels are inserted into the grid of uprights and crossbars and kept in place by pressing elements or structural sealants.