The rule is between 0.4pt and 0.5pt
\starttext \dontleavehmode \scale[factor=200]{$1 \over 2$} \scale[factor=200]{$1 \above .47pt 2$} \stoptext
Lovely! This applies to ConTeXt's default font size of 12pt, doesn't it? Hence we would have the following linear relation if the thickness is 0.4pt at 10pt font size: thickness = 0.4pt * fontsize/10pt. Judging from the following: --- \startuseMPgraphic{rule} linecap := butt; defaultrulethickness := 0.4pt*BodyFontSize/10pt; path p; p := origin--(EmWidth,0); draw p withpen pencircle scaled defaultrulethickness; \stopuseMPgraphic \starttext Lorem ipsum dolor sit amet \startformula \sum_{n=0}^\infty \frac{x^n}{n!}\!\vcenter{\useMPgraphic{rule}} \stopformula \setupbodyfont[32pt] Lorem ipsum dolor sit amet \startformula \sum_{n=0}^\infty \frac{x^n}{n!}\!\vcenter{\useMPgraphic{rule}} \stopformula \setupbodyfont[72pt] Lorem ipsum dolor sit amet \startformula \sum_{n=0}^\infty \frac{x^n}{n!}\!\vcenter{\useMPgraphic{rule}} \stopformula \stoptext --- this seems to be true in general. (Hans, Taco, can you confirm this officially?) Could one perhaps pass "DefaultRuleThickness" on to MetaFun just as the variables "BodyFontSize", "ExHeight" etc. are made available? Best, Oliver