Thank you, it's almost working. I am making use of projects, products, environments and components. My \usemodule[ruby] sits in my environment file, but I'm using the \ruby command down in the component file. The environment is only called in the project file, but the component links back to the product (with \product - I guess that's how you do it?) and the product back to the project. The error occurs, no matter at which level (project/product/component) I am compiling. Do I really need to call the module in each component? I thought the component should trace back to the higher levels to find which modules to load... Many thanks for your quick answers, Severin

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I am looking to add furigana (readings) above the Kanji (Chinese characters) in a Japanese text.

In XeTeX I'd be used to something translating to

\usemodule[simplefonts] \usemodule[ruby] \starttext \ruby{??}{????}? \stoptext

What would be the correct command for mkiv?

You wrote it yourself above but you need at least a font with japanese characters.

Wolfgang