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How does italic correction work?

  1. Where are the correction values looked up?
  2. Why is it not automatically included?
  3. What is the effect of multiple \/ or \/ in odd places such as between letters or after a space?

(There is a related question about italic correction as described in the TeXbook.)

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This is a property of the font. Every character of the font has four associated lengths specified in the tfm (font metric) file. Height, Width, Depth and Italic Correction.

1 When you use \/ TeX extracts the dimension from the font metric file, scales it by any scaling that has been applied to the font and then inserts that much horizontal space.

2 Often you need aesthetic judgement around punctuation as to whether it is needed. (But LaTeX \textrm and similar command do apply it automatically and get it mostly right.)

3 Well the correction spacing is applied, and also any ligatures or kerns that would be between those letters are lost. (Originally I added most of the time here).

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  • most of the time? When are ligatures not lost?
    – Martin Schröder
    Jan 15, 2013 at 20:46
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    @MartinSchröder 20 years ago I'd have constructed an example by now, something in the back of my brain made me add that rider but I can't get it to fail, I thought it was if tex did a third emergencystretch hyphenation pass and the italic correction was zero it got lost and so theligature was re-constructed, but I don't have the conditions quite right in my head at present:-)
    – David Carlisle
    Jan 15, 2013 at 20:55
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    @MartinSchröder my mind was playing tricks on me, the TeX2 Book suggested using {} to break ligatures, that does fail after hyphenation and \/ is a suggested fix TeX(3) book says 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 anitalic font); shelf{\kern0pt}ful is often best.
    – David Carlisle
    Jan 15, 2013 at 21:44
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    Yes, \/ looks at the previous item (in the list being typeset) and if that is a character it looks up the italic correction, for a second \/ the previous item will be a kern from the first one, so later ones don't see a character and so do nothing.
    – David Carlisle
    Jan 17, 2013 at 11:25
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    @LoverofStructure Don Knuth's phrase not mine that was a quote from the TeXBook, if you go shelf{}ful TeX initially puts two f into the horizontal list not an ff ligature but when considering hyphenation TeX takes apart ligatures (so it can look up the word in its hyphenation trie) and re-constitutes the ligatures if a hyphenation doesn't happen, but it "forgets" that the ligature was disabled and reconstitutes the ff ligature if shelfful is considered as a possible break point but not broken.
    – David Carlisle
    Jan 19, 2013 at 0:12

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