If you're shopping for a bed, you may wonder, "What is a box spring?" Box springs are a common component of any bed set. But many new mattresses claim not to need this under-mattress support.

When you buy a mattress online, it’s most likely going to arrive tightly rolled and vacuum-wrapped in plastic, inside a large cardboard box. This is what is called a bed in a box mattress.

Most bed-in-a-box mattresses are made from foam or a mix of foam and springs. Foam mattresses are great if you like a bed ...