There are some entities not currently meeting the definition of country in ISO 3166-1 listed in ISO 3166 due to the history of the standard, and therefore are legacy. However, over the past years, a significant effort has been made to only include countries that have been recognized by the United Nations. Moreover, due to the limited number of two-letter combinations, the ISO 3166/MA assigns country codes very carefully since once a two-letter country code element is assigned and then deleted, it cannot be re-used for 50 years.