[texhax] Blank output with just \hrule

[Philip TAYLOR]

Ludwig Mises wrote:
> I know I've read about this in the TeXbook but I can't seem to find it
> now.  I have the following:
> 
> \hrule
> \hrule
> \hrule
> \hrule
> 
> \bye
> 
> This results in blank output, yet if I insert a word anywhere between
> any of those hrules, suddenly all 4 hrules show up. 

Odd, I don't exactly see that : if I write

> \hrule
> \hrule a
> \hrule
> \hrule
> 
> \bye

I see two hrules, not four.  One just above
the "a", one below.  These are fatter than normal
hrules, so I suspect I am seeing two near
co-incident pairs,  but I can't prove this
visually.  If I type

> \leavevmode
> \hrule
> \hrule
> \hrule
> \hrule
> 
> \bye

then I see one big fat hrule, clearly fatter than
I see after

> \leavevmode
> \hrule
> 
> \bye

hence supporting the "near co-incident pairs" hypothesis.

Anyhow, I think the simple answer to your question
is that the \hrules have zero width in the absence
of something to trigger TeX into typesetting one
or more line boxes.  Test this using

> \hrule width 6 pc
> \hrule width 6 pc
> \hrule width 6 pc
> \hrule width 6 pc
> 
> \bye


Philip TAYLOR