ISO 3166-1 alpha-2 is a widely used country code system, along with what three-letter system?
ISO 3166-1 alpha-2:  They are the most widely used of the country codes published by ISO (the others being alpha-3 and numeric), and are used most prominently for the Internet's country code top-level domains (with a few exceptions). ISO 3166-1 alpha-3: ISO 3166-1 alpha-3 codes are three-letter country codes defined in ISO 3166-1, part of the ISO 3166 standard published by the International Organization for Standardization (ISO), to represent countries, dependent territories, and special areas of geographical interest.
ISO 3166-1 alpha-3