ISO 3166-3 is part of the ISO 3166 standard published by the International Organization for Standardization (ISO), and defines codes for country names which have been deleted from ISO 3166-1 since its first publication in 1974. The official name of the standard is Codes for the representation of names of countries and their subdivisions – Part 3: Code for formerly used names of countries. It was first published in 1999.
Each former country name in ISO 3166-3 is assigned a four-letter alphabetic code. The first two letters are the ISO 3166-1 alpha-2 code of the former country, while the last two letters are allocated according to the following rules:
Besides the former country name and its ISO 3166-3 code, each entry in ISO 3166-3 also contains its former ISO 3166-1 codes, its period of validity, and the new country names and ISO 3166-1 codes used after its deletion from ISO 3166-1.
After a country is deleted from ISO 3166-1, its alpha-2 and alpha-3 codes will be transitionally reserved for a transitional period of at least five years. After the expiration of the transitional period, these codes are free to be reassigned.