The default dimensions of rules are defined in chapter 21 of the
The TeXbook. To sum up what is given there, the "thickness"
of rules is
0.4pt by default. Eplain defines three parameters that let you change
this dimension: \hruledefaultheight, \hruledefaultdepth,
and \vruledefaultwidth. By default, they are defined as
The TeXbook describes.
But it would be wrong to redefine \hrule and \vrule. For
one thing, some macros in plain TeX depend on the default dimensions
being used; for another, rules are used quite heavily, and the
performance impact of making it a macro can be noticeable. Therefore,
to take advantage of the default rule parameters, you must use
\ehrule
and \evrule.