The four possible quartet topologies. The four possible quartet topologies of species a, b, c, d. Topologies (a): ab|cd, (b): ac|bd, and (c): ad|bc are butterfly quartets, while topology (d): ab×cd MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeGadiahceaGeaapguaabeqaceaaaeaacqWGHbqyaeaacqWGIbGyaaGaey41aqBbaeqabiqaaaqaaiabdogaJbqaaiabdsgaKbaaaaa@3501@, is a star quartet.