The systems depicted in (A) and (B) have an upper critical solution temperature (UCST) and a lower critical solution temperature (LCST), respectively. (C) shows a system with both a UCST and an LCST, where the two-phase regime exists between these critical temperatures, resulting in a closed loop. (D) shows a system with both a UCST and an LCST, with a two-phase regime above the UCST and below the LCST, resulting in an hourglass shape.