Bed sizes vary considerably around the world, with most countries having their own standards and terminology.
While the double size appears to be standard among English speaking countries, based on the imperial measurement of 4 ft 6 in by 6 ft 3 in (137,16 cm x 190,5 cm), the sizes for other bed types tend to vary. The European sizes differ, not merely because of difference based on use of the metric system.
A king-sized bed differs from the other sizes in implementation, as it is not common to have a king-sized box spring; rather, two smaller box-springs are used under a king-sized mattress. It is a common misconception that on a U.S. standard or eastern king, the box springs are identical in size to a twin extra-long; however, twin extra-long mattresses next to each other add up to 78 inches wide instead of the 76 inch width that is standard for an eastern king.
U.S. terminology refers to a twin bed to mean what is known as a single bed in other countries, whereas other countries understand twin beds to be two single beds in the same room.