One thing I meant to say, is if \mathsf{abc} is implemented by switching to a sans font. What to do about $ U+1D5BA U+1D5BB U+1D5BC $ that is MATHEMATICAL SANS-SERIF SMALL [ABC] The choices (either justifiable) would be to let them go through to the sans math alphabet in the base font or to give them mathcodes with ascii codepoints and the \fam of the sans serif font so that it typeset as \mathsf{abc} Unlike the switching in the other direction this would not need a mathcode switching loop in every instance you could just set the mathcodes once, at the point the mathsf family was allocated. Switching to use a text font is I think closest to the intended spirit of these characters and their use in MathML. <mi mathvariant="sans-serif">abc</mi> is defined to be equivalent to <mi>𝖺𝖻𝖼</mi> and distinct from <mi>𝖺</mi><mi>𝖻</mi><mi>𝖼</mi> That is it's defined to be distinct although whether or not it is visually distinct naturally depends on the fonts in use. Thus MathML (at least) expects to be able to use runs of plane 1 characters as multi-letter identifiers and have them typeset as such, it doesn't mandate (or disallow) that the characters go straight through to font slots with that index. David