1911 Encyclopedia Britannica
In architecture, a method of vaulting used in the Perpendicular style, of which the earliest example is found in the cloisters of Gloucester cathedral, built towards the close of the 14th century. The ribs are all of one curve and equidistant, and their divergency, resembling that of an open fan, has suggested the name. One of the finest examples, though of later date (1640), is the vault over the staircase of Christ Church, Oxford. For the origin of its development see Vault.
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Chisholm, Hugh, General Editor. Entry for 'Fan Vault'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/f/fan-vault.html. 1910.