(1) \definefont[myChapterFont][myHeadingsFont sa 1.728] You could either use a real font name like 'Arial' or a symbolic name defined with \definefontsynonym like 'Serif', 'SerifBold', 'Sans', 'Dummy-Regular' etc. in the second argument of \definefont but not the name for a font collection defined with \definetype, in your case \definefont[myChapterFont][Sans sa 1.728] or \definefont[myChapterFont][DummyRegular sa 1.728]
So ( and considering the example at http://wiki.contextgarden.net/Chapters_headers ) I tried... \setuppapersize [A4][A4] \starttypescript [serif] [myBodyFont] \definefontsynonym [myBodyFontRegular] [name:Times New Roman] [features=default] \definefontsynonym [myBodyFontItalic] [name:Times New Roman Italic] [features=default] \definefontsynonym [myBodyFontBold] [name:Times New Roman Bold] [features=default] \definefontsynonym [myBodyFontBoldItalic] [name:Times New Roman Bold Italic] [features=default] \stoptypescript \starttypescript [serif] [myBodyFont] \setups[font:fallback:serif] \definefontsynonym [Serif] [myBodyFontRegular] \definefontsynonym [SerifItalic] [myBodyFontItalic] \definefontsynonym [SerifBold] [myBodyFontBold] \definefontsynonym [SerifBoldItalic] [myBodyFontBoldItalic] \stoptypescript \starttypescript [myBodyFont] \definetypeface [myBodyFont] [rm] [serif] [myBodyFont] [default] \stoptypescript \starttypescript [sans] [myHeadingsFont] \definefontsynonym [myHeadingsFontRegular] [name:Arial] [features=default] \definefontsynonym [myHeadingsFontItalic] [name:Arial Italic] [features=default] \definefontsynonym [myHeadingsFontBold] [name:Arial Bold] [features=default] \definefontsynonym [myHeadingsFontBoldItalic] [name:Arial Bold Italic] [features=default] \stoptypescript \starttypescript [sans] [myHeadingsFont] \setups[font:fallback:sans] \definefontsynonym [Sans] [myHeadingsFontRegular] \definefontsynonym [SansItalic] [myHeadingsFontItalic] \definefontsynonym [SansBold] [myHeadingsFontBold] \definefontsynonym [SansBoldItalic] [myHeadingsFontBoldItalic] \stoptypescript \starttypescript [myHeadingsFont] \definetypeface [myHeadingsFont] [ss] [sans] [myHeadingsFont] [default] \stoptypescript \usetypescript[myBodyFont] \usetypescript[myHeadingsFont] \setupbodyfont[myBodyFont,12pt] % this works: body font displays in the correct font %\definefont[myChapterFont][myHeadingsFontRegular sa 1.728] % compile error at line 51: WHY ? \definefont[myChapterFont][Arial sa 1.728] % only this works: WHY ? %\definefont[myChapterFont][Sans sa 1.728] % defaults to default LM font for line 51: WHY ? %\setuphead[chapter][style=myChapterFont] \setuphead[chapter][style=\myChapterFont] \starttext \chapter{chapter test} % line 51 \section{section test} text test \stoptext Many thanks in advance for your enlightening help. Alan