consensus Call: TATWEEL

John C Klensin klensin at
Mon Mar 23 22:06:31 CET 2009

--On Tuesday, March 24, 2009 01:06 +0430 Alireza Saleh
<saleh at> wrote:

> I think this is the registry  choice to make it DISALLOW.
> Would you  please tell me the reason of using  low-rise or
> Hyphen in English and  why it is allowed both for DNS and IDN ?

That decision was, for all practical purposes, made around 1971.
The intention was to have a way to divide mnemonics and
structure names well before the DNS came along.  So, for
example, we had "MIT-Multics" (note the hyphen) which evolved
into "Multics.MIT.EDU".  The hyphen was then included in the
DNS, at least in large part, in order to support the transition
strategy which migrated from "MIT-Multics" (standalone host name
and in the host table) to MIT-Multics.ARPA" (transitional DNS
name) to "Multics.MIT.EDU" (target DNS name).   I have no idea
how many people thought of it as transitional in the DNS
(gradually disappearing as we shifted from host-table names to
DNS ones) or a permanent features.

Low-line (underscore) has never been permitted, partially
because of visual confusion issues with hyphen (there is at
least one other reason, but it can be easily explained only in
terms of 1403 print trains :-(  ).

And hyphen is a full character in Latin script, going back
centuries, and not used for justification, calligraphy, or
typography only.  For some applications of the latter, there are
other characters to represent the EMdash and ENdash... and they
are not permitted in IDNs either.

If there are lessons in the hyphen, they are that one should be
quite careful to avoid, or at least consider the implications
of, transition strategies that are likely to last forever.


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