What do you call this kind of comforter holder?

Question:It's some sort of mechanism, that is fastened to the bed or mattress, but supposedly you don't see it. It allows you to turn down your comforter or quilt to the end of the bed, but holds it in place so it doesn't fall off the end. I've heard of them, but don't know what to call it to even search for one.

Answers:
Not sure if this is what you're looking for.