Ligatures and hyphenations: the effect of empty brace groups {}

Question

For the following example,

\hsize =2in

shelf{}ful shelf{}ful shelf{}ful
rot{}ten rot{}ten
shelf{}ful shelf{}ful shelf{}ful

shelf{}ful shelf{}ful
rot{}ten rot{}ten
shelf{}ful shelf{}ful

\bye

The result is as followings:

My questions is: in the first paragraph, why the first shelfful is without ligature, but the other shelfful are with ligatures?

Answer 1

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.