\starttext

    \setupbars[unit=mm,rulethickness=1]     bar\startbar[underbar]foo\stopbar bar\blank
    \setupbars[unit=ex,rulethickness=1]     bar\startbar[underbar]foo\stopbar bar\blank
    \setupbars[unit=pt,rulethickness=1]     bar\startbar[underbar]foo\stopbar bar\blank
    \setupbars[unit=pt,rulethickness=10pt]  bar\startbar[underbar]foo\stopbar bar\blank

\stoptext
