Hi Hans, there is \dimexpr missing otherwise \mapfontsize has no effect. %\def\font_basics_set_mapped_fontsize#from% % {\ifcsname\??mappedfontsize#from\endcsname % \csname\??mappedfontsize#from\endcsname\else#from% % \fi} \def\font_basics_set_mapped_fontsize#from% {\ifcsname\??mappedfontsize\the\dimexpr#from\relax\endcsname \csname\??mappedfontsize\the\dimexpr#from\relax\endcsname\else#from% \fi} Example: \mapfontsize[12pt][24pt] \starttext 12pt\par \definedfont[Serif mo 1]12pt \stoptext Wolfgang