e”. The TeX output stream is in a somewhat different situation: characters in it are to be used to select glyphs from the fonts to be used. Thus the encoding of the output stream is notionally a font encoding (though the font in question may be a virtual font). In principle, a fair bit of what appears in the output stream could be direct transcription of what arrived in the input, but the output stream also contains the product of commands in the input, and translations of the input such as ligatures like
fi. Font encodings became a hot topic when the Cork encoding appeared, because of the possibility of suppressing
accentcommands in the output stream (and hence improving the quality of the hyphenation of text in inflected languages, which is interrupted by the
accentcommands — see “how does hyphenation work”). To take advantage of the diacriticised characters represented in the fonts, it is necessary to arrange that whenever the command sequence “
e” has been input (explicitly, or implicitly via the sort of mapping of input mentioned above), the character that codes the position of the “é” glyph is used. Thus we could have the odd arrangement that the diacriticised character in the TeX input stream is translated into TeX commands that would generate something looking like the input character; this sequence of TeX commands is then translated back again into a single diacriticised glyph as the output is created. This is in fact precisely what the LaTeX packages inputenc and fontenc do, if operated in tandem on (most) characters in the ISO Latin-1 input encoding and the T1 font encoding. At first sight, it seems eccentric to have the first package do a thing, and the second precisely undo it, but it doesn’t always happen that way: most font encodings can’t match the corresponding input encoding nearly so well, and the two packages provide the sort of symmetry the LaTeX system needs.
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This is FAQ version 3.26, released on 2013-02-25.