[texhax] Blank output with just \hrule

[Ludwig Mises]

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.  Will someone
point me to the explanation (if possible)?  I tried to put them in a
\vbox but that made no difference (why would it since we're already in
vertical mode right)?

Thanks,

Ludwig