Conjoined twins

Some pairs, depending on the degree of conjunction--in particular, the degree to which they share internal organs--can be separated by surgery. A famous pair, Chang and Eng (1811-1874), were known as the Siamese Twins, and this name has often been used as a synonym for conjoined twins.

In some cases, parts of the brain have been known to be shared between conjoined twins joined at the head.