Full and double are two different names that are used to describe the same size bed. Because the two names are still in popular use, mattress companies often refer to them interchangeably. Interesting factoid: Up until the 1940s, the two most popular mattress sizes were twin or double (full). In the 1950s, queen and king size beds were introduced to consumers as alternative choices to a double bed and by 1999, queen size mattresses became more popular than full size.