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Q. I understand a space is necessary between a number and a fraction when the fraction symbol is unavailable (e.g., 2 1/2), because the number would be illegible without it. But what if you use the symbol?
A. Fractional quantities expressed as a numeral plus a symbol are normally written without a space, as in 2½ or 5⅞. See CMOS 9.15 for examples.
Whether the symbol is used or not, these are known as “vulgar” fractions. For example, the Unicode name for “⅞” is “vulgar fraction seven eighths.” In this context, “vulgar” means “common.” We can only guess, then, that decimal fractions (e.g., 0.875) would be considered fancy by comparison.