[OS X TeX] ligaturess

[cfrees at imapmail.org]

On Sun 18th Jul, 2010 at 06:15, Jonathan Kew seems to have written:

>>>>
>>>> Why does TeX ignore the instruction not to use the ff ligature in
>>>> 'shelful' when a line break occurs afterwards? And why does it
>>>> nonetheless accept the instruction if the f-no-lig-f is typeset in
>>>> isolation?
>>>
>>> Because it attempted hyphenation, while looking for (and failing to find) a better way to line-break the paragraph.
>>>
>>> {} is not a reliable way to prevent a ligature. Use {\kern0pt} or possibly \/. See The TeXbook, p.306.
>>
>> Thank you. (I should probably look at The TeXbook sometime...)
>>
>> So the compwordmark gets discarded as part of attempting hyphenation?
>> Interesting.
>
> {} is not a "compound word mark". It is a "nothing": simply an empty group in your TeX input. As such, it disappears during the process of reading your input and generating the "horizontal list" that subsequently forms the lines of your paragraph; however, it does interrupt the formation of a ligature *at that initial stage*.

Oh. That's useful to know.

The reason I thought as I did is that t1.etx says:
---t1.etx---
...
\setslot{compwordmark}
    \comment{An invisible glyph, with zero width and depth, but the
       height of lowercase letters without ascenders.
       It is used to stop ligaturing in words like `shelf{}ful'.}
\endsetslot
...
---end---
So I assumed that, at least if T1 was active, including {} was
including a real character - albeit an invisible one with zero width.
Which was why the output I got didn't make sense to me.

Thanks,
cfr