This dissertation uncovers three previously unrecognized innovations of Thomas Jeckyll in the Peacock Room. At the same time, the dissertation admits that sometimes James McNeill Whistler chose a more conventional path in the design of the room than previously acknowledged. The dissertation illuminates the often overlooked principle of Classical Decor, first described in the first century BC by Vitruvius, and analyzes how it was instituted in the Peacock Room. Four major points illustrate this conclusion. First, the meaning of the sunflower in the West is explored to account for the flower’s popularity and absorption into ancient heliotropic lore. Thomas Moore’s poetry may have inspired Aesthetic Movement designers such as Jeckyll to use the motif. Second, this dissertation demonstrates that the Peacock Room is only a distant descendant of the traditional European porcelain chamber. It was a new idea to turn the porcelain chamber into a dining room. Further, the room lacks two of the three key features of a porcelain room: lacquer panels and large plate-glass mirrors. When Whistler made the surfaces of this room dark and glossy, he made the room more traditional, aligning it with the customary lacquer paneling of porcelain rooms. And Jeckyll’s sho-dana shelving system in the Leyland dining room was without precedent in porcelain or other kinds of Western rooms, with influences from Japan and China. Third, Decor in the dining room was revealed as an established pattern in eating rooms from Ancient Roman triclinia to the present day. Fourth, Decor is present in the Peacock Room in four ways: in the trappings of the table used to decorate a dining room, in the darkness of this dining room, in the use of a foodstuff, the peacock, to decorate the room, and in the hearth’s sunflowers. Through the lens of the history of Western domestic interiors, significant innovations by Jeckyll have been brought to light, and the meaning of specific elements in the Peacock Room has been elucidated. Jeckyll and Whistler gave the world a sensational story in the Peacock Room but also a complex work of art that is only beginning to be illuminated.