diff --git a/README.md b/README.md
index db7936c..3c6046e 100644
--- a/README.md
+++ b/README.md
@@ -56,7 +56,7 @@ Below are the essential tags for basic, minimalist websites:
 <!-- Geo tags -->
 <meta name="ICBM" content="latitude, longitude" />
 <meta name="geo.position" content="latitude, longitude" />
-<meta name="geo.region" content="country[-state]" /><!-- Country code: mandatory, state code: optional; eg. content="US" / content="US-NY" -->
+<meta name="geo.region" content="country[-state]" /><!-- Country code (ISO 3166-1): mandatory, state code (ISO 3166-2): optional; eg. content="US" / content="US-NY" -->
 <meta name="geo.placename" content="city/town" /><!-- eg. content="New York City" -->
 ```