The most famous pair of conjoined twins was Chang and Eng Bunker (1811-1874), Thai brothers born in Siam, now Thailand. They traveled with P.T. Barnum's circus for many years and were billed as the Siamese Twins. Chang and Eng were joined by a band of flesh, cartilage, and their fused livers at the torso. Due to the brothers' fame and the rarity of the condition, the term came to be used as a synonym for conjoined twins. (Photo by: Wellcome Collection/Pictures From History/Universal Images Group via Getty Images)