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Mistake #5: Forgetting the 18-Inch Margin

Reason why people do it: People want to make a rug they already own work in a different space, or they fall in love with the color or pattern of a new one without first considering its size.

Reason why it's a mistake: A too-large rug will make a room feel even more cramped. Too small? The furniture starts to feel as if it's floating higgly-piggly in the space.

What to do instead: In a large space, you want 15 to 20 inches between the rug edge and the wall. In a smaller room, leave an 8- to 14-inch perimeter. (All four legs of the seating furniture should be on the rug, unless the sofa is against a wall).

One extra tip to keep in mind: In a good-sized room, the ideal margin is 18 inches; to get a custom-looking rug without killing your budget, have a piece of carpet cut to the dimensions you need and bind the edges with a matching wool or nylon yarn.