For a [[monster girl]], such as a [[mermaid]] or [[lamia]] whose legs are fused into a tail, but only starting somewhere below the waist, leaving the thighs still separated at the top.  

At minimum, there must be enough room to run a string from front to back through the middle.  Having an [[ass]] or [[pussy|vagina]] alone isn't enough.

This is frequently part of an artist's attempt to solve the so-called "mermaid problem."

h4. See also
[[Forked Tail]] : One tail that splits into two around midway. 
split tail