Implications of Logical Order Exception Property

Richard Wordingham richard.wordingham at
Wed Jan 25 13:10:15 CST 2017

On Wed, 25 Jan 2017 08:13:10 +0000
Richard Wordingham <richard.wordingham at> wrote:

> What is the significance of logical_order_exception being true for a
> character?
> TUS 9.0 Section 4.3 appears to claim that such characters need to be
> rearranged 'logically' for searching and sorting.
> However, I cannot see how they need to be rearranged for searching.
> Is this property a general warning, or does it mean that swapping with
> the *next* character gives a less bad sorting experience?  For
> example, why doesn't U+17CC KHMER SIGN ROBAT have the property?
> Consonant + ROBAT has to be rearranged to ROBAT + consonant for
> sorting - ROBAT is a repha stored after rather than before the
> *visual* base consonant.

After some further research, I see that it is relevant to Revisions 9
and 11 of the Unicode Collation Algorithm.  For earlier and later
revisions, its effects were defined by the collation element table
rather than the Unicode Character Database.  The reordering for ROBAT
is the wrong way round for the property to be applicable in its
original meaning.  I can't find any other formal requirement for the

I now have a clutch of errors to report on Unicode's use of the term
'logical order' and references to logical_order_exception:

1) Claims that Thai is not encoded in logical order in
  Technical Report 10 (Collation)
  UCD file IndicPositionalCategory.txt
2) Claims that logical_order_exception is relevant for searching (TUS,
as above) 

Should I make this one report or three reports?


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