The authors prove the authenticity of the Boston counterpart to the Ludovisi Throne. A careful physical examination of the surface of the Boston counterpart was made, and the Ludovisi Throne was also examined. Action of ground water, root marks, previous cleaning, a little recutting, surface layers, and patina containing fragile shells of land snails were all detected. A natural weathering layer was observed and photographed in thin section. The marble was compared chemically and crystallographically with other Greek marbles and the conclusion drawn that the marble of both reliefs came from a quarry on Thasos. A Roman reshaping of both reliefs also seems likely. All doubts of the authenticity of the Boston counterpart are dismissed and both reliefs are declared to be genuine Greek works dating from 470-460 BC.