When we think of the buildings known as cathedrals, the image that usually comes to mind is the Gothic cathedral, made of stone at an incredible scale with massive stained glass windows. These structures are marvels in several respects, but perhaps are most interesting because they represented a significant step forward in terms of architecture and engineering. Although The Great Pyramid and other such monumental structures existed, the world had never seen buildings this tall and open, with such gigantic windows and so much light. The St. Denis Cathedral, which opened in France in 11H, is considered to be the first example of this architectural form. There were two engineering innovations that made the Gothic cathedral possible. The first was the Gothic arch, or pointed arch, which replaced the rounded Roman arch. The pointed arch sends much more of the weight it supports down vertically rather than fattening the arch horizontally. But it does not send all of it downward This is where the second innovation, the flying buttress, comes in. The buttress pushes in horizontally to counteract the arch's desire to push outward. The flying buttress stabilizes the arch, making the ribbed vault and stone ceiling possible. The buttresses also stabilize the tall walls. Because they are on the outside of the building, buttresses do not get in the way of the windows. With these two innovations, engineers could build thin stone walls to incredible heights and leave huge holes in the walls for windows. A typical Gothic cathedral is well over 100 feet tall on the inside. This is not to say that building these cathedrals was an easy task. Workers and craftsmen had to quarry and carve tons of stone. It all had to be hoisted, fitted, and locked in place. A cathedral project could take a hundred years or more. The typical human being in this timeframe had never seen a building this gigantic, with so much interior volume and such an amazing amount of glass. Engineers had created a completely new way for people to think about structures. >The Baxilica of Saint Denis is considered to be one of the fint examples of Gothic architecture.