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NRSI: Computers & Writing Systems

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You are here: Encoding > Unicode
Short URL: http://scripts.sil.org/CharStories_2024

Character Stories: U+2024 ONE DOT LEADER

[Source: Ken Whistler, Unicore list 2003-5-30]

U+2024 ONE DOT LEADER is a graphic character, whose glyph consists of a small baseline dot, and whose General Category is Po (Other Punctuation). It cannot be used conformantly as if it were a formatting control standing in for a rich text representation of a leader object (e.g. in a generated Table of Contents in a Word or FrameMaker document)…

Now, here is the true story of U+2024.

It is a compatibility character, introduced for compatibility with XCCS (Xerox Character Code Standard) 1980, where it was mapped to the coded character 356B/242B (0xEEA2), described as "Leader, one-dot on an en body".

Its use in XCCS would have been to create leaders manually, by lining up a sequence of "one-dot on an en body" to create a sufficiently long leader. Its rationale in Unicode would be to either map to data created in XCCS or to manually lay out text using a comparable mechanism, but for which one wished to distinguish the "dots" thus used from U+002E FULL STOP.

U+2025 TWO DOT LEADER is also an XCCS compatibility character. It corresponds to XCCS 356B/243B (0xEEA3) "Leader, two-dot on an en body" *and* to 041B/105B (0x2145) "Leader, two-dot on an em body". The difference in width was considered a formatting distinction and was unified away in creating the U+2025 encoded character, as preserving that distinction in plain text was considered unnecessary by the Xerox representative to the committee at the time.

U+2026 HORIZONTAL ELLIPSIS maps to the ellipsis seen in a number of legacy character encodings, including the Macintosh character sets, but also maps to an XCCS character: 041B/104B (0x2144) "Leader, three-dot on an em body".

All *three* of these characters should be considered compatibility characters. Indeed, they formally *are* "compatibility decomposable characters" (Chapter 3, Definition D21), since they each have compatibility decompositions to one or more U+002E FULL STOP characters.

That last fact should be taken as a hint that for most purposes, manual leaders should just be sequences of FULL STOP characters (as you will see, for instance in the plain text representations of Internet Drafts or RFCs, for example). But in any rich text format, leaders are styled formatting objects (somewhat similar to tabulations, as Philippe suggested), but that does *not* make U+2024 a format character (LEADER PLACEHOLDER, or whatever). It is exactly what it claims to be: a compatibility character, punctuation, with a single baseline dot as its glyph.



[From a subsequent message in the same thread:]

> Why then do you insist that [U+2024] represents one dot ?

Because that was the intent of the Unicode Technical Committee when it encoded the character, and is the clear intent of the standard as currently specified.



[From a subsequent message in the same thread:]

> As a typesetter on Mac OS X, I see no reason to abandon the use of

> the three-dotted horizontal ellipsis character, Ken.

Nor do I. It is fine for ellipses...

And it was encoded for that. But in encodings which don't have an ellipsis character, it is roughly comparable to a sequence of three periods, as above. And for sure you wouldn't want to create leader lines by using a sequence of ellipses.


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 Reply
"Peter", Fri, Jun 12, 2009 17:22 (CDT)

One dot leader also plays a role of Armenian stop (mijaket)

According to Unicode standard (page 248), Armenian stop called mijaket is represented as a one dot leader U+2024

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