They are characterized by their two mouths – a normal one located on her face and a second one on the back of the head beneath the hair. There, the woman's skull splits apart, forming lips, teeth and a tongue, creating an entirely functional second mouth.
In Japanese mythology and folklore, the futakuchi-onna belongs to the same class of stories as the rokurokubi, kuchisake-onna and the yama-uba, women afflicted with a curse or supernatural disease that transforms them into yōkai. The supernatural nature of the women in these stories is usually concealed until the last minute, when the true self is revealed.
The nukekubi are rokurokubi whose heads come off and float about. These were the last of the rokurokubi whereas the other kind came before. Nukekubi sometimes perform bad deeds such as attacking at night and drinking their victims' blood. It's theorized that the nukekubi has a weakness when it is sleeping and the head is floating around: if the body moves, then the head cannot be re-united to the body. Classical literature about rokurokubi describe tales of people witnessing and encountering floating heads at night time.