A four-centered arch is a low, wide type of arch with a pointed apex. Its structure is achieved by drafting two arcs which rise steeply from each springing point (centre) on a small radius, and then turning into two arches with a wide radius and much lower springing point. It is a pointed sub-type of the general flattened depressed arch.
In the picture above, the lower two centres line up with the pillars, but this is not necessary, you can experiment by changing the four centres. The upper 2 should lie on the line between the top of the supporting pillars. The next picture shows a variation with a resultingly lower arch.
Two of the most notable types are known as the Persian arch, which is moderately "depressed", and the Tudor arch, which is much flatter.
Tudor arch at Layer Marney Tower, 1520s
It is a mainstay of Islamic architecture, especially Persian and Mughal architecture; in the latter the lowest arches often have scalloped edges. In Asia it typically has a less depressed form than in "Tudor"-style examples. T
The picture above shows four centred arches used in the Taj Mahal
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