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A mattress pad, mattress topper, or underpad is designed to lie atop a mattress. Made primarily from foam, its function is to provide a layer of comfort and warmth between the user and the mattress. It is of particular benefit when the existing mattress is worn or uncomfortable. Toppers quite often have a contoured profile to facilitate airflow. Visco-elastic foam toppers are regularly used in medical institutions because of their pressure relieving properties. The foam used in the construction of these memory foam toppers reacts to both pressure and temperature. This, combined with its greater density, are properties that give them their characteristic "weightlessness" sensation.
An electric mattress pad is made of heavy fabric with heating filaments woven into it. In the UK and Commonwealth countries this is called an "electric blanket".