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Pediment: the (usually triangular) upper part of a gable in an ancient building, often decorated with a relief.
The pediment of an ancient building is the (usually triangular) upper part of the gable, often decorated with a relief, which is called the tympanum. This relief can be very elaborate: for example, the reliefs of the western and eastern pediments of the temple of Aphaea on the island Aegina, the early and late pediments of the temple of Apollo in Delphi, the pediments of the temple of Zeus in Olympia, and the eastern and western pediments of the Parthenon in Athens (the "Elgin Marbles").
In the Hellenistic and Roman periods, there we semicircular pediments as well.