CUSP, technically, the intersection of two curves; hence, in architecture, the intersections of lobed or scalloped forms, par ticularly in arches (cusped arches) and tracery. Thus the three lobes of a trefoil (clover leaf form) are separated by three cusps. Cusped forms appear commonly in early Mohammedan work (see MOHAMMEDAN ARCHITECTURE) , as in the mosque of Ibn Touloun at Cairo (c. 875), and were especially frequent in the Moorish work of Algiers and Spain. The cusp is found occasion ally in the Romanesque work (see BYZANTINE AND ROMANESQUE ARCHITECTURE) of France, as in the chapel of St. Michel de l'Aiguille at Le Puy en Velay (probably late i ith century), due perhaps to influence from Spain, but it did not become popular until the Gothic period (see GOTHIC ARCHITECTURE), during which it was used universally and frequently enriched with leaves, flowers or even human heads at the tip.