Ischiopagi is the medical term used for conjoined twins (Class V) united at the pelvis.
It specifically describes a condition involving the fused lower half of the two bodies, with spines conjoined end-to-end at a 180° angle. Ischiopagic twins have four arms; two, three or four legs; and typically one external genitalia and anus.[1]
Ischiopagus Dipus: the twins share two legs
Ischiopagus Tripus: the twins share three legs, the third leg is often two fused legs, or is non-functioning; only one set of external genitalia is common
Ischiopagus Tetrapus/Quadripus: the twins have four legs