Objects characterized by curving lines and shapes, such as arcs or circles. Throughout history the circle has often been viewed as a symbol of perfection and completion, while the S-curve, named after the mathematical function from which it results, has been considered supremely beautiful, its gentle flow forming the basis of poses for figures since the Classical period. The 18th century English painter William Hogarth developed an aesthetic theory around this so-called “Line of Beauty”, declaring it the liveliest of shapes, and included a depiction of it, carved in wood, in his self portrait of 1745. Important instances of curvilinear forms in the modern period include Brazilian architect Oscar Niemeyer's sensuously curved futuristic buildings cast in concrete, and Frank Lloyd Wright's Solomon R. Guggenheim museum building in New York (1943), which spirals upward and outward from the base. While regular curvilinear forms appear to be created with mathematical precision, irregular ones may appear shaky, organic, or even biomorphic.