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TeX Frequently Asked Questions
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For example

! Improper \hyphenation will be flushed.
\'#1->{
       \accent 19 #1}
<*> \hyphenation{Ji-m\'e
                        -nez}

(in Plain TeX) or

! Improper \hyphenation will be flushed.
\leavevmode ->\unhbox 
                      \voidb@x 
<*> \hyphenation{Ji-m\'e
                        -nez}

in LaTeX.

As mentioned in “hyphenation failures”, “words” containing \accent commands may not be hyphenated. As a result, any such word is deemed improper in a \hyphenation command.

Hyphenation happens as paragraphs are laid out; by this time, TeX knows what font is used for each glyph; thus it knows the encoding being used. So the solution to the problem is to use a font that contains the accented character; doing this this “hides” the accent from the hyphenation mechanisms.

For LaTeX users, this is quite an easy task; they select an 8-bit font with the package, as in \usepackage[T1]{fontenc}, and accented-letter commands such as the \’e in \hyphenation{Ji-m\’e-nez} automatically become the single accented character by the time the hyphenation gets to look at it.