The Probert Encyclopaedia of Architecture

CUSP

In architecture a cusp is a triangular protection from the intrados of an arch, or from an inner curve of tracery. Cusps came into use during the latter part of the Early English style, at which period they were sometimes worked with a small leaf, usually a trefoil, on the end. When first introduced, the cusps sprung from the flat under surface or soffit of the arch entirely independent of the mouldings, and this method was sometimes followed in Decorated work; but they very soon began to be formed from the inner moulding next to the soffit (usually either a splay or a hollow,) and this continued to be the general practice until the expiration of Gothic architecture.

In the Decorated and Perpendicular styles, they were frequently ornamented at the ends, either with heads, leaves, or flowers, and occasionally with animals. A few varieties in the mode of forming cusps may occasionally be met with; in the chancel of Solihull church, Warwickshire, which is of early Decorated date, the arcs of the featherings, instead of uniting in a point in the usual way, terminate in small curls; also at the bay windows in the hall of Eltham palace, Kent, which is late Perpendicular work.
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