That's weather stripping. When you close the doors, they rest on top of the strip. Are you buying that? How about-- it's a track for the optional sliding screen door that Bryce Depot has on special order for me? Is that more to your liking? Ummm- let's see. The house shifted and the marble floor conveninetly cracked at the doorway? Black Painter's tape? The architect is an idiot and made the outer marble deck higher than the floor so the rain will pour into the house because he has a lot of stock in a local mop company? Actually, it's a boolean error on my part. There is a very thin cut-out for the outer cube that makes up the rest of the room (it's too complicated to explain why I had to do it this way) and it is sticking through the floor because I forgot to make the floor a Boolean NEUTRAL and it is grouped to the outer cube (I forget why). One has to watch those boolean values!