body-text-and-readinglisted

# Body text and reading Running text is the only place where typography is measured against a physical process: the eye's saccadic movement. Every rule here derives from that, not from taste. ## 1. When to invoke - Setting body copy for anything read in succession (>1 paragraph). - A reader reports strain, re-reading lines, or losing their place. - Choosing or changing body size, line-height, or column width. - Auditing an existing article/doc template. **Do not** invoke for isolated UI strings, labels, or numeric displays — that is `ui-component-typography`. Do not invoke for headings alone — that is `hierarchy-and-scale`. ## 2. Required context Refuse to give a number until you have, or have explicitly assumed: - The **typeface** and its x-height (apparent size varies enormously at equal nominal size). - **Target device and reading distance** (phone ~35cm, laptop ~50–70cm, TV ~3m, kiosk standing). - **Content type** — reference/scanned vs. immersive/continuous. - **Audience** — age, vision, whether Dynamic Type / user zoom is expected. - **Available column width**, and whether it is fixed or fluid. If you have none of these, say what you're assuming before you answer. ## 3. Invariant principles - **Information is absorbed only during fixations, not during motion.** The eye moves in saccades of roughly 5–10 letters (~1–2 English words) alternating with fixations of 0.2–0.4 seconds. Only 3–4 letters are focused sharply; the rest are perceived indistinctly