Why do some doorways have a thin strip of marble on the ground (beneath the door, when closed.)?

Question:I've seen this a lot. In fact, in my home there are several instances of this and *no other* marble anywhere else in the house. Does it serve some function?

Answers:
It's called a transition strip, or threshold, and it does serve a purpose. If you have a room that has ceramic tile in the floor, and the room that is next to it has hardwood flooring, the threshold serves to act as a transition between the two different types of flooring, beacuse they would not look good if they butted up against each other. Plus they are also two different thicknesses, so you would see a difference in the height, and you could stub your toe or hurt your feet. Hope that helps