Request to register private-use variant subtags

Doug Ewell doug at
Sat Apr 7 23:10:40 CEST 2012

Gordon P. Hemsley wrote:

>> Though I note that there would be a difference between such putative
>> private use variant subtags and the -x- mechanism: everything
>> (including other singletons) after -x- is private use, that would not
>> be the case for private use variant subtags.
> This is an important point that I hadn't even thought of. Private use
> subtags allow for localized private use—that is, for only a particular
> type of subtag. I think there is a difference between a private-use
> variant (= a variant that just so happens to not be registered) and a
> private-use -x- extension (= could be anything under the sun, like the
> '-x-ignore-this-subtag' that I've also used in my tests).

A subtag of any registered type (language, extlang, script, region, 
variant) that "just so happens to not be registered" is invalid for use 
in any tag or language-range. Testing is another matter; you are 
strongly encouraged to test your application or engine against 
non-registered subtags, to ensure that they are properly detected as 


>> I don't see why we should encode subtags for testing purposes. For
>> personal use OR for corporate use.
> Just so I'm clear: If it were up to you, there wouldn't be private-use
> subtags for language, region, or script, either?

1. Michael didn't say that.

2. As I did say last week, private-use subtags for language, region, and 
script aren't for testing purposes. They are based on code elements in 
ISO 639, 3166, and 15924 respectively, which were primarily intended for 
people using those standards directly. They can also be used in BCP 47, 
but with the assumption that there is a private agreement that governs 
their use, not to mean "nothing" or "invalid."

More information on the intended use of subtags like 'qaa' is available 
in RFC 5646, sections 2.2.1 (point 3), 2.2.3 (point 3), 2.2.4 (point 3), 
and 4.6, as well as the following passage from Section 2.2.9:

"Subtags designated for private use as well as private use sequences 
introduced by the 'x' subtag are available for cases in which no 
assigned subtags are available and registration is not a suitable 
option.  For example, one might use a tag such as "no-QQ", where 'QQ' is 
one of a range of private use ISO 3166-1 codes to indicate an otherwise 
undefined region."

Doug Ewell | Thornton, Colorado, USA | @DougEwell ­ 

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