Everyone knows what a Gothic arch looks like. It is an arch with a point. There are many variations on the pointed arch theme. There are formulas as to how to make these arches, all with various shapes and names including Lancet, Equilateral, Ogee and Reverse Ogee Arches. The point is they are not rounded at the top.
The Gothic arch came about after the Roman arch in about 1120. It has two advantages; in that a Gothic arch reaches higher for a given width and it will produce only half the side-thrust of a similar Roman arch . A major consideration when building a masonry arch is the amount of horizontal thrust that it produces on its foundations. Masonry walls can easily absorb large vertical compressive forces. A Gothic arch sends the force downward more vertically.
Where the Gothic arch came from is another question. It is often described as an invention of medieval French masons, as if was just dreamed up from nowhere. Some people say the pointed arch was a graft on the Romanesque, Lombard, and Byzantine architecture of Europe. It's interesting to note however that the pointed vault was developed in Persia many years before. The actual first appearance of the pointed arch in the Muslim World has been traced to the Al-Aqsa Mosque built in the late 700's . Some scholars think it was this mosque that inspired the Crusaders to imitate the Muslim pointed arch in Europe.
No matter where it came from, the crazy thing is that the Gothic arch came after the Roman arch. In a later blog I'll explain why I think this is so strange.