# LaTeX hyphenation different for inline optional hyphenation and explicit \hyphenation{…} definition

In a text, I get an overfull hbox. To avoid this, I define an optional hyphenation point:

bla bla bla un\-known bla bla bla


pdflatex indeed hyphenates the word at that point. But when I declare the optional hyphenation point in the preamble like this

\hyphenation{un-known}


then pdflatex will NOT hyphenate there. The reason seems to be that LaTeX still thinks that the typeset text does not look good enough with the line break after 'un-'. But then why is the behavior different for the inline optional hyphenation to the optional hyphenation declared in the preamble?

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Are you using multiple languages with babel? – egreg Apr 7 '12 at 15:45
If you can process your document with LuaTeX: include the package lua-visual-debug and you'll see the possible hyphenation points. – topskip Apr 7 '12 at 16:07

Try adding \showthe\lefthyphenmin so TeX stops and shows the value of this parameter. If it is set more than 2 then automatic hyphenation will never break after 2 letters. The default (for English) is 2, but some language setups change this.
No babel package used; but \showthe\lefthyphemin did the trick. It was set to 3. After setting it to 2 (simply use \lefthyphenmin=2), my optional hyphenation work as expected. Thanks for the hint. – Klaus Apr 9 '12 at 12:19