# Re: Dotless j (was RE: FW: ISO entity names)

From: J%ORG KNAPPEN (KNAPPEN@ALPHA.NTP.SPRINGER.DE)
Date: Wed Apr 05 2000 - 12:35:38 EDT

From a TeXnicians point of view, the LATIN SMALL LETTER DOTLESS J is a
character, not a glyph. The distinction is quite clear and firm in TeX.
It is already in good old plain TeX, not in a macro package like AMSTeX
or LaTeX2e.

Excursion: On characters and glyphs in TeX

The TeX system consists of TeX the program and TeX fonts (implemented in
METAFONT). In the TeX fonts we have glyphs, in TeX we have either direct access
to them (for the most popular ACSII characters) or indirect access via control
sequences. The set of direct accessible characters and control sequences defines
the character repertoire of TeX. It is not coded, since no numbers are assigned.
(The glyphs in the fonts are accessed by numbers, not by glyph names, but this
is another story to be told another time).

In principle, all glyphs are accessible, but in practice several of them
don't have control sequences. The TeX fonts contain for example a glyph called
"mapstochar" which is only used to build mathematical arrows like "|---->".
Only the ready-build arrow has a control sequence ("\mapsto" in our example).
The pieces of which it is build don't have control sequences.

End of Excursion

The dotless j is accesible directly as \j in TeX for text and as \jmath for
math, it is therefore a character. Note, that it is not necessary in TeX, that
\j is a character in order to build accented versions of it. In esperant.sty,
the j with circumflex is entered simply as "^j" and TeX does the right thing
internally. The same is true for LaTeX2e in respect to accented versions of
the letter i---\^i gives î as expected. In plain TeX, the same input gave an
i with dot and circumflex above.

Similarly, the capital letter scharfes s is a character in LaTeX2e (\SS), but
not available in plain TeX. LaTeX2e supports some other characters, plain TeX
didn't know (Thorn, Eng, D with bar, Edh).

--J"org Knappen

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