Full

A full measures 54 by 75 inches: sixteen inches wider than a twin and not one inch longer. Older listings and most of the world call it a double, which is the same mattress under a second name.

That second name causes most of the trouble. Double suggests two sleepers, and the arithmetic disagrees. Split down the middle, each person gets 27 inches, which is less than the 38 inches a child gets alone in a twin bunk. Couples on a full are sharing space, not dividing it comfortably.

For one person it is generous. Fifty-four inches allows a diagonal sprawl, a laptop beside you, and a dog on the bed without negotiation. Length is the unchanged catch: at 75 inches, anyone near 6 feet meets the end of the bed exactly as they would on a twin.

Rooms usually make the decision. A full fits where a queen does not, and in a bedroom under about 10 by 11 feet it leaves walking space a queen would eat. Frames, headboards and bedding all price below their queen equivalents. Resale and hand-me-down value is the soft spot, since most couples move up to queen and most guest rooms already own twins.