Monospace sized by a guessed ratio
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Rule · bucket: judgment · severity: note · detector: none · since: v0.9.0
Section: Things to avoid
Group: Typography

Forbidden:
  Inline `code` set to `0.9em` — or any fixed ratio — to stop it looking bigger than the text around it

Instead:
  Measure both faces before correcting either. If they share an x-height, the ratio is the mismatch. Let code inherit its host's size.

The habit comes from pairings where it is true: a mono face drawn separately from the text face often does sit larger at the same `font-size`. In a superfamily it does not. IBM Plex Sans and IBM Plex Mono are both x-height 51.6 and cap-height 69.8 per 1000 units — identical — and mono is *narrower*, not larger. A `0.9em` there sets code at x-height 46.8 inside text at 52, creating the mismatch it was meant to remove.
A ratio is also the wrong shape of answer. Inline `code` appears inside body text, headings, table cells and captions; one multiplier has to be right for all of them, and a fixed token is worse still — it collapses code in a heading to caption size. Inheriting is correct in every host, which is why this rule has no token.
The measurement is one line in a browser: render `x` in both faces at the same size and compare the rendered heights, or read `sxHeight` from each font's `OS/2` table. Do it once per project when the brand file is written, not per component.