The TeXBook has this exercise:
exercise Sometimes you run into a rare word like 'shelfful' that
looks better as shelf{}ful
without the ff
ligature. How can you
fool TeX into thinking that there aren't two consecutive f's in such
a word?
answer {shelf}ful
or shelf{}ful
, etc.; or even
shelf\/ful
, which yields a shelf/ful instead of a
shelf{\kern0pt}ful. In fact, the latter idea---to insert an italic
correction---is preferable because TeX will ^reinsert the ff
ligature by itself after hyphenating shelf{}ful
. Appendix H
points out that ligatures are put into a hyphenated word that contains
no explicit kerns, and an italic correction is an explicit
kern.) But the italic correction may be too much (especially in an
italic font); shelf{\kern0pt}ful
is often best.
Note here that "after hyphenating" should be read as "after trying to hyphenate"
If TeX attempts to hyphenate a word it essentially splits it up at the possible
break points to add discretionary hyphens. If then hyphenation is not used at that point the word is reconstituted without those hyphenation points, but the original {}
information is lost by then and ligatures will be formed in the reconstituted word.
TeX (not luatex) never attempts to hyphenate the first word of a paragraph (essentially a bug that was classified as a feature to keep TeX stable) and also it does not attempt to hyphenate any words in a paragraph if it can break the paragraph into lines and keep within \pretolerance
badness. So whether f{}f
actually breaks an ff ligature is rather variable depending on the whole paragraph, not just the word where {}
is used.
{}
seem to do nothing, since theff
are ligatured. – Manuel Oct 28 '14 at 10:15{}
to break ligatures for precisely this reason, the{}
is lost if the word is reconstituted after a hyphenation trial. – David Carlisle Oct 28 '14 at 10:25