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

Question

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?

Answer 1

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.