Exercise 11.6 of The TeXbook describes a macro \frac
for setting fractions, but \frac never made it into plain TeX.
So Eplain includes it.
\frac typesets the numerator and denominator in
\scriptfont0, slightly raised and lowered. The numerator and
denominator are separated by a slash. The denominator must be enclosed
in braces if it's more than one token long, but the numerator need not
be. (This is a consequence of \frac taking delimited arguments;
see page 203 of The TeXbook for an explanation of
delimited macro arguments.)
For example, \frac 23/{64} turns `23/64' into
.