[texhax] Em width (Was re: Overfull boxes)

[Joel C. Salomon]

On Wed, Aug 1, 2012 at 12:03 PM, Philip TAYLOR <P.Taylor at rhul.ac.uk> wrote:
> Joel C. Salomon wrote:
>> That's not to say that the character called the "em dash" will be
>> exactly 1 em long; font designers can play all sorts of typographical
>> games. But TeX, when given lengths measured in ems, will use the size
>> of the font.
>
> Not always :
>
> \font \xviiirm = cmr10 at 18 true pt
> \xviiirm
> \dimen 0 = 1 em
> \message {1 em = \the \dimen 0 }
> \end
>
> (./untitled-1.tex 1 em = 18.00005pt )
>
>
> \font \xviiirm = cmtt10 at 18 true pt
> \xviiirm
> \dimen 0 = 1 em
> \message {1 em = \the \dimen 0 }
> \end
>
> (./untitled-1.tex 1 em = 18.89983pt )

I stand corrected. But—

What's going on with the example you gave? Why is this so?

—Joel