From Londonhua WIKI
Vaulting is used to hold the roof up when a large open space is needed. There are several types of Vaulting with each having its own unique aspects. Some are more basic while others are more decorative.
Background or Origin of Article
A Barrel Vault is an arched shape, similar to the roof of a tunnel. As the upper parts of the arch push the lower sections out, something must be resisting that outward pressure. In a tunnel the fill around the tunnel does this, but above ground this is done with thick walls that support it.
Ribbed Vaulting is masonary framework of intersecting arches, or ribs. This type of Vaulting has been used in the late Norman and Gothic periods. 
Fan Vaults have ribs that are equal in length and radiate out from a single point. Shorter ribs cross the radiating ribs creating extravagant patterns.
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