\starttext \font\test={file:linlibertine_r.otf:+frac} \test 1/2 3/4 5/6 7/8 9/10 11/12 31415/27182 1000/1000000 \stoptext gives as output ½ 3¾ 5⅚ 7⅜ 9/10 1⅟12 31415/27182 1000/1000000 (I couldn't build an example without the primitive command. After \definefontfeature[frac][frac=yes] \definefontfamily[default][rm][linuslibertineo][features=frac] context always falls back to latin modern) A similar plaintex example for xetex \font\test="[linlibertine_r.otf]:+frac" \test 1/2 3/4 5/6 7/8 9/10 11/12 31415/27182 1000/1000000 \bye gives the correct result: ¹⁄₂ ³⁄₄ ⁵⁄₆ ⁷⁄₈ ⁹⁄₁₀ ¹¹⁄₁₂ ³¹⁴¹⁵⁄₂₇₁₈₂ ¹⁰⁰⁰⁄₁₀₀₀₀₀₀ A plain tex example for luatex is broken too but differently to the context version, one gets at least the correct numbers: \input luaotfload.sty \font\test={file:linlibertine_r.otf:+frac} \test 1/2 3/4 5/6 7/8 9/10 11/12 31415/27182 1000/1000000 \bye ½ ¾ ⅚ ⅞ 9/10 1⅟12 31415/27182 1000/1000000 -- Ulrike Fischer http://www.troubleshooting-tex.de/