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1911 Encyclopedia Britannica
(Lat. cuspis, a spear, point), a projecting point, or pointed end. In architecture (Fr. feuille, Ital. cuspide, Ger. Knopfe), a cusp is the point where the foliations of tracery intersect. The earliest example of a plain cusp is probably that at Pythagoras school, at Cambridge, - of an ornamented cusp at Ely cathedral, where a small roll, with a rosette at the end, is formed at the termination of a cusp. In the later styles the terminations of the cusps were more richly decorated; they also sometimes terminate not only in leaves or foliages, but in rosettes, heads and other fanciful ornaments. The term "feathering" is used of the junction of the foliated cusps in window tracery, but is usually restricted to those cases where it is ornamented with foliage, &c.
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Chisholm, Hugh, General Editor. Entry for 'Cusp'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/c/cusp.html. 1910.
the Week of Proper 9 / Ordinary 14