[texhax] could someone give me an explanation as to why fontsize fails with an overfull box

[Carlos]

On Sun, Jan 27, 2019 at 09:16:20PM +0000, David Carlisle wrote:
>> \overfull boxes all over, regardless whether the \hsize is set to
>> negative values or not, and not \underfull boxes, as I wrongly assumed
>> it would have been the case with a negative value in place.
>
>Naturally every line is overfull if you set a target width for line
>breaking of -2in.
>even an empty line is then overfull by 2in and any text in the line
>makes it more overfull .
>
>TeX can not possibly make any sensible output with such a setting.
>

Then it follows, based on that statement, that every line is underfull
if I were to set a target width for line breaking of a positive value.

Even a non-empty line would be underfull and any text would make it
more underfull. 

Thank you.

Carlos