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