Question: ISO 3166-1 alpha-2 is a widely used country code system, along with what three-letter system?
Sentence (from "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).
Relevant evidence: Yes