The double mattress, or full mattress, actually provides less room per person than a twin mattress.
In the U.S., a double mattress is 54 inches wide and 75 inches long, according to the consumer website Mattress Sizes.
According to the website of The Better Sleep Council, double mattresses are only big enough for one adult.
Standard twin mattresses are 39 inches wide and 75 inches long, according to Mattress Sizes. If a double mattress is shared, it provides only 27 inches of space per person. According to The Better Sleep Council, this is about the width of a crib.
While a double mattress might not be the most comfortable choice for couples or tall people, they are useful for children's rooms, providing children with ample space for stretching out, according to The Better Sleep Council.
The Better Sleep Council recommends a king-size bed for adult couples. King beds are 76 inches wide and 80 inches long.