# [texhax] The Em Rule, a Question of Style

Wed May 19 15:51:42 CEST 2010

```On Wed, May 19, 2010 at 11:43:43AM +0100, Philip TAYLOR wrote:

> P. R. Stanley wrote:
>
> >Thanks for your response. So, would you say that mbox and the ~ are
> >interchangeable in this instance?
>
> Without looking at the definition of \mbox one cannot be
> certain, but I would hypothesise that \mbox creates a
> single indivisible unit that cannot possibly be split
> unless it is \un[*]box'd, whilst the tie ("~") simply
> inhibits splitting at one particular place, allowing
> other potential breakpoints to still have their
> intended effect.

A space, also ~ have stretch/shrink components. They get
lost, if they are put in the box.

"~\mbox{--}~" instead of "\mbox{ -- }" is therefore better.
The dash creates a breakpoint by its own with the assoziated
penalty \exhyphenpenalty. \mbox prevents the breakpoint here
regardless of the setting of \exhyphenpenalty.

Yours sincerely
Heiko Oberdiek
```