[groff] 13/14: groff_char(7): Clarify _, \_, \[ul].

[G. Branden Robinson]

gbranden pushed a commit to branch master
in repository groff.

commit 1627cbb538c7435c218f52898a48d33e0352c18d
Author: G. Branden Robinson <g.branden.robinson@gmail.com>
AuthorDate: Fri May 7 23:49:24 2021 +1000

    groff_char(7): Clarify _, \_, \[ul].
    
    * Discuss escape sequences that are converted to special characters on
      input.  (Once only, then forget about them, especially in the tables.
      Except for \-, which damnably resists all simplifying discussions.)
    * Document distinction between _ and \[ul].
    * Fix error (introduced on 6 May) in table row for _.
---
 man/groff_char.7.man | 39 +++++++++++++++++++++++++++------------
 1 file changed, 27 insertions(+), 12 deletions(-)

diff --git a/man/groff_char.7.man b/man/groff_char.7.man
index 5b56845..eae1687 100644
--- a/man/groff_char.7.man
+++ b/man/groff_char.7.man
@@ -118,15 +118,26 @@ The driver used in generation of this page was 
\[lq]\*[.T]\[rq].
 .
 .
 .P
-A few escape sequences that are not special character escapes also
-produce glyphs;
+A few escape sequences that are not
+.I groff
+special characters also produce glyphs;
 these exist for syntactical or historical reasons.
 .
-They include
-.BR \e\e ,
 .BR \e\[aq] ,
 .BR \e\[ga] ,
 .BR \e\- ,
+and
+.B \e_
+are translated on input to the special characters
+.BR \e[aq] ,
+.BR \e[ga] ,
+.BR \e[-] ,
+and
+.BR \e[ul] ,
+respectively.
+.
+Others include
+.BR \e\e ,
 .B \e.\&
 (backslash-dot),
 and
@@ -134,13 +145,6 @@ and
 see
 .IR groff (@MAN7EXT@).
 .
-Of these,
-only
-.B \e\-
-is also available as a special character of the same name,
-in the form
-.BR \e[\-] .
-.
 A small number of special characters represent glyphs that are not
 encoded in Unicode;
 examples include the baseline rule
@@ -1323,6 +1327,17 @@ The Unicode-derived names of these three glyphs are 
approximations.
 .
 .
 .P
+The input character
+.B _
+always accesses the underscore glyph in a font;
+.\" unless one isn't available, but this seems to be only a theoretical
+.\" concern--what font doesn't support every ASCII codepoint these days?
+.BR \[rs][ul] ,
+by contrast,
+may be font-invariant on typesetter devices.
+.
+.
+.P
 The baseline rule
 .B \[rs][ru]
 is a font-invariant glyph,
@@ -1357,7 +1372,7 @@ _
 |      |       u007C   bar
 \[ba]  \e[ba]  u007C   bar
 \[br]  \e[br]  u2502   box rule +
-\&\_   \&_     u005F   underscore, low line +
+\&_    \&_     u005F   underscore, low line +
 \[ul]  \e[ul]  u005F   underscore, low line +
 \[rn]  \e[rn]  u203E   root extension, overline +
 \[ru]  \e[ru]  ---     baseline rule +