# Why is hyphenation not tried for words followed by em-dash?

Consider the following mystory.tex:

\tolerance=10000 \hsize=.7in
\leavevmode\nobreak\hskip0pt Drofnats---or R. J.,''
\end


This is the log:

(mystory.tex
Overfull \hbox (17.41133pt too wide) in paragraph at lines 2--3
[] \tenrm Drofnats---|
[1] )


This is the output:

Now change mystory.tex by adding space between "Drofnats" and "---".

This is the log:

(mystory.tex
Underfull \hbox (badness 10000) in paragraph at lines 2--3
[] \tenrm Drof-

Underfull \hbox (badness 10000) in paragraph at lines 2--3
\tenrm nats ---or
[1] )


This is the output:

As far as I understand typographic rules dictate that we must use em-dash like word1---word2. Do the typographic rules also prohibit hyphenation of word1? If not, why hyphenation is not tried by default?

• hyphenation is not tried in words containing an explicit hyphen, or ligatures ending in a hyphen, you can use a macro that surrounds the --- with 0pt space if you want to allow hyphenation – David Carlisle Jan 3 at 12:12
• Is anyone even using word---word anymore? I though even the British use word -- word these days – daleif Jan 3 at 12:13
• @daleif or something like \,---\, – David Carlisle Jan 3 at 12:14
• @DavidCarlisle 1) "\,---\," - but \, works only in math mode. 2) Using \kern0pt before --- will add hyphenation to "Drofnats" without allowing break before --- (as by default); in contrast, using \hskip0pt before --- will add hyphenation to "Drofnats" by allowing break before ---. 3) Do you know where to find exact typographic standards? (like using spaces around --- or using , before closing '' etc) – Igor Liferenko Jan 4 at 9:53
• The em dash is often used without spaces (Chigago style) but I find munch more readable a em dash with spaces (Associated Press style) that moreover avoid this problem. – Fran Jan 4 at 9:56

⟨letter⟩, ⟨otherchar⟩, \char⟨8-bit number⟩, ⟨chardef token⟩, \noboundary. The most common commands of all are the character commands that tell TeX to append a character to the current horizontal list, using the current font. [...] In unrestricted horizontal mode, a ‘\discretionary{}{}{}’ item is appended after a character whose code is the \hyphenchar of its font, or after a ligature formed from a sequence that ends with such a character.
Since --- is a ligature that ends with the \hyphenchar of the current font, that's usually the hyphen, a \discretionary{}{}{} item is automatically appended and TeX never hyphenates words containing a \discretionary item (see page 454, end of last but one double dangerous bend).