Postal codes in the Netherlands, known in Dutch as postcodes, were introduced in 1977 by the then PTT, now PostNL. They are alphanumeric, consisting of four digits followed by two uppercase letters. The letters 'F', 'I', 'O', 'Q', 'U' and 'Y' were originally not used for technical reasons, but almost all existing combinations are now used as these letters were allowed for new locations from 2005 on. The letter combinations 'SS', 'SD' and 'SA' are not used because of their associations with the Schutzstaffel, the Sicherheitsdienst and the Sturmabteilung respectively before and during the 1940-45 Nazi occupation of the Netherlands.