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"Cusp, in architecture, is the point formed by the meeting of two small arches, or foils, in foil arches, or tracery. Cusps often terminate in rich bosses of flowers and leaves." — Chambers' Encyclopedia, 1875

Cusp

"Cusp, in architecture, is the point formed by the meeting of two small arches, or foils, in foil arches,…

In architecture, a piercing or panel divided by cusps or foliations into four leaves, or more correctly the leaf shaped figure formed by the cusps. It is supposed to represent the four leaves of a cruciform plant.

Quatrefoil

In architecture, a piercing or panel divided by cusps or foliations into four leaves, or more correctly…