[texhax] TeX Queries 4: Overfull boxes

[Paul Stanley]

Hi
extract from TeXbook, chapter 6:

[start quote]
\begindisplay
\hbox spread-1em{You don't get spaces between words narrower than 
this,\ and}\cr
\hbox spread+1.679895em{you don't get spaces between words wider than this.}\cr
\enddisplay
[end quote]

Does the above depend on the size of the font? Presumably the 
physical width of em varies from font to font.

What is a box?
Does it refer to an imaginary box encapsulating the line? What does `\hbox' do?
Anything else the list considers important about overfull boxes?

I'll leave "underfull" for another thread, unless the list considers 
it relevant.

many thanks,
Paul