An important characteristic of Gothic church architecture is its height, both absolute and in proportion to its width, the verticality suggesting an aspiration to Heaven. The increasing height of cathedrals over the Gothic period was accompanied by an increasing proportion of the wall devoted to windows, until, by the late Gothic, the interiors became like cages of glass. This was made possible by the development of the flying buttress, which transferred the thrust of the weight of the roof to the supports outside the walls. As a result, the walls gradually became thinner and higher, and masonry was replaced with glass. The four-part elevation of the naves of early Cathedrals such as Notre-Dame (arcade, tribune, triforium, clerestory) was transformed in the choir of Beauvais Cathedral to very tall arcades, a thin triforium, and soaring windows up to the roof.