Riddle me this...

Why is it that a mermaid is described as half-women, half-fish, when their tails are clearly that of a mammal and not that of a fish? A fish's tail moves left to right, whilst a mammals tail goes up and down.

This is the only mermaid I've ever seen with a fish tail and not a mammal's tail.

Spoiler:

because they are really more human than fish (use of half isn't literal exactly) and the way the human body is the mammal arrangement makes far more sense

Also...saw HP6 last week...and tbh the cloak worked just like it should...it didn't protect him from direct attacks...but hid him...When malfoy body-bound him he fell down and malfoy just pulled it off...and all luna did was terminate the curse at which point he pulled it off

Well, the tail looks like a fish tail, and mermaids live underwater...thus, the fish part. And is that really what happened in the movie? I'm not a diehard fan of the books, but the movies should at least attempt to keep the scenes the same.

Dark, Malfoy didn't pull the cloak off - he used a spell and it flew off. He only pulled it back on. I never said that it protects Harry from spells, only that it protects him from being found.

Also, Dark, my comment wasn't on that it was half and half, it's the fact that it's compared to a fish. It's not half-women-half-dolphin or whale, dugong, etc, it's half fish. The picture I've attached is the only mermaid I have seen which matches the description - ie, has a fish half. Most have mammal tails.

My dog's tail goes side to side...

In terms of water-borne mammals, silly.

The fish tail is attached after being rotated 90 degrees...most fish aren't wider then they are (measure between belly and back)

It doesn't function like a fishes tail, it functions like a dolphin's tail. The also don't have the fins needed for support like fish do. They aren't fish tails rotated 90 degrees. They are based on the dugong, or, that's where the folklore started - but I'm just asking why it's termed "half-fish".

Admitedly, being half-fish isn't a very strong design in terms of performance in the water. The mermaid in Harry Potter move really awkwardly.

It's called "half-fish" because the tail looks like a fish. Until now, I had never, ever considered that the tail moved differently. And I don't think many other people did...or any, for that matter.

that's 3mpty for you...anyway...the reason the tail moves up and down is because of the human element...It makes the body look more normal(erest) and it allow for a far smoother transformation if your dealing with a mermaid that can shift between human and merhuman