Very good point by the artist Kantoku. The tail is an extension of the spine, and should extend down, right above the anus. This would indeed require a modification to the underwear.
There are many drawings where the tail of a cat-ear character emerges from the back slightly above the panties, but if you compare with a real cat I think it should be slightly lower? Wouldn't you need a hole in the panties? Hence this drawing. The Bengal, American Shorthair, and Russian blue are drawn exactly like my own cats!