HTTP: The Definitive Guide

G.2 Second Subtag Rules

If the second subtag is:

                Two characters long, it's a country/region defined by ISO 3166^[3]

^[3] The country codes AA, QM-QZ, XA-XZ and ZZ are reserved by ISO 3166 as user-assigned codes. These must not be used to form language tags.

                Three to eight characters long, it may be registered with the IANA

                One character long, it is illegal

The ISO 3166 country codes are summarized in Table G-3.