# 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