ISO 3166 country codes
Also called: iso 3166, country codes, region codes, iso 3166-1 alpha-2
ISO 3166 country codes are the international standard for naming countries and territories in short form. Part one (ISO 3166-1) gives each of 249 entries a two-letter alpha-2 code (US, GB, DE), a three-letter alpha-3 code, and a three-digit numeric code. SEO uses the alpha-2 codes in hreflang region tags.
Three parts make up the family. ISO 3166-1 covers current countries and territories (the codes you actually type). ISO 3166-2 handles subdivisions like states and provinces. ISO 3166-3 records codes for countries that no longer exist. For search, only 3166-1 alpha-2 matters, and it shows up in one place: the region subtag of an hreflang annotation.
Where SEO actually uses them
Google is strict about the format. An hreflang value is a language code from ISO 639-1, optionally followed by a dash and a region code from ISO 3166-1 Alpha 2, like en-US. You cannot ship a region on its own. Google says plainly that it will not derive the language from a country code, so a bare us reads as a language and gets ignored.
Two traps quietly eat traffic. The United Kingdom is GB, never UK (UK is reserved but not assigned to a country). And invented regionals like es-419 are not on the ISO list, so they are unsupported. When a code is invalid, Google drops the whole annotation without an error in the interface, which is why the fix rarely gets prioritized.
The list is not frozen either. The ISO 3166 Maintenance Agency assigns and retires codes as UN membership changes, so a hardcoded lookup table can drift out of date over the years.
How it affects your traffic
For a store selling into multiple countries, these two-letter codes are what tell Google which page belongs to which market. Get one wrong (an en-UK tag, or a region with no language in front of it) and Google discards the mapping, so your German page can outrank your US page for US shoppers, or the wrong currency and shipping copy surfaces in search. On a large catalog that silent breakage multiplies across thousands of URLs and leaks conversions market by market. Getting the codes and hreflang right at the template level is core E-commerce SEO work, because it protects the market targeting you are already paying to build.
Get E-commerce SEO that moves the needle
We turn terms like this into ranked pages and qualified pipeline. Start with a free Initial SEO Strategy.