A standard twin mattress is 39 inches wide and 75 inches long. A length of 75 inches is often too short for many adults, which is why the twin XL was invented. A twin XL is 39 inches wide and 80 inches long.
A full-size bed is sometimes called a double bed. In comparison to a twin, it is 53 inches wide and 75 inches long. This means that although a full-size mattress is wider than a twin, it still may be uncomfortable for taller adults.
Twin beds became popular in the 1890s because, at that time, doctors were convinced that full-size beds were unsanitary and that people who slept in these beds would suffer injury at one time or another.