--- \usetypescript[cambria] \setupbodyfont[cambria]
\def\defaultrulethickness{\the\fontdimen8\textfont3\relax}
\starttext
\the\defaultrulethickness
\stoptext ---
which used to work in the old MkII days.
That is with the first two lines commented out :-)
I was finally able to come up with this: --- \usetypescript[cambria] \setupbodyfont[cambria] \starttext FractionRuleThickness (in display style): \the\Umathfractionrule\displaystyle FractionRuleThickness (in script style): \the\Umathfractionrule\scriptstyle etc. \stoptext --- This should work with any font in MkIV. More parameters can be found in the LuaTeX manual at http://www.luatex.org/svn/trunk/manual/luatexref-t.pdf in section 5.4 on pages 114f. (Boy, did it take me time to discover this! Seriously.) Oliver