Bed skirt

A bed skirt or valance is a piece of decorative fabric that is placed between the mattress and the box spring of a bed. The purpose of a bed skirt is to give a stylish appearance to a bed without exposing the sides of the box spring or any space under the bed that may be used for storage. Historically, bed skirts were used to block drafts, which could chill the undersides of beds, and to prevent dust from accumulating under the bed.

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Wikipedia