The Gothic rib vault

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A vault built as a skeleton of stone arches - the ribs - with thin panels of masonry (the webs or severies) filling between them. Two things follow. First, the pointed arch used for the ribs can be made steeper or shallower at will, so ribs of different span can be brought to the same crown height; a Romanesque semicircular vault cannot do that, which is why Romanesque bays tend to be square and Gothic bays need not be. Second, the ribs can be erected on relatively light centring and the webs built afterwards, which cuts the cost of the timber formwork that dominated vault construction. Whether the ribs actually carry the vault, or mainly stiffen and organise it while the shell carries itself, has been argued since the 19th century and is not settled. Either way the vault concentrates its load onto the piers, and the wall between the piers stops being structural - which is the opening through which the glass arrives.