23AF HORIZONTAL LINE EXTENSION: glyph or variation selector?

Jukka K. Korpela jkorpela at cs.tut.fi
Wed Apr 2 14:39:10 CDT 2014

2014-04-02 21:56, Whistler, Ken wrote:

> U+23AF is *definitely* not a variation selector at all.
> It is part of a set of bracket pieces (and other graphic pieces)
> in the range U+239B..U+23B1.
> These glyphic pieces of symbols are only relevant and useful
> in the context of mathematical typesetting programs like TeX.

I’m not sure whether TeX uses such characters at all. TeX is oriented 
towards typesetting glyphs, often not caring that much about abstract 
characters. When I use, say, $$\begin{pmatrix}…\end{pmatrix} in LaTeX to 
get a nicely formatted array with large parentheses around, I don’t 
think LaTeX internally uses characters like U+239B.

On the other hand, such characters can be used in very primitive 
“typesetting” in a plain text environment under some conditions. For 
example, to create a largish left parentheses I could use U+239B U+239C 
… U+239C U+239D each at the start of a new line:


This won’t work on everyone’s email reader, of course. It works in 
Notepad, for example. On a web page, it works when you set the text 
solid, with line-height: 1. Of course, there would be the issue of font 
coverage, but I don’t see any particular reason why such characters 
could not be used in plain text, in word processors, in HTML 
documents—apart from the practical point that there are usually better 

U+23AF is a simpler building block, but it has its problems, too. 
Despite the purpose mentioned in a comment in the standard, there is no 
guarantee that it joins smoothly with adjacent simple arrows. But of 
course it is a graphic character, and one that can be expected to have a 
rather specific shape. It’s not something abstract that says that some 
arrow should be extended; rather, it can be used as an extension.


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