Parapets are either plain, embattled, perforated or panelled. The last two are found in all styles except the Romanesque.
Plain parapets are simply portions of the wall generally overhanging a little, with a coping at the top and corbel table below.
Embattled parapets are sometimes panelled, but oftener pierced for the discharge of arrows, etc.
Perforated parapets are pierced in various devices as circles, trefoils, quatrefoils and other designs so that the light is seen through.
Panelled parapets are those ornamented by a series of panels, either oblong or square, and more or less enriched, but not perforated. These are common in the Decorated and Perpendicular periods.
Compare breastwork.
Original text from the 1911 Encyclopaedia Britannica