Measure (line length)
How many characters fit on one line of text. Around 45 to 75 reads comfortably; a paragraph spanning your whole monitor does not.
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What it is
Measure is how long a line of text is, counted in characters rather than pixels. The book-typography rule of thumb is 45 to 75 characters per line for body copy, with 66 as the classic sweet spot (Robert Bringhurst's number). Too long and your eye loses its place on the return sweep to the next line. Too short and you get choppy rhythm plus a ragged, hyphen-heavy edge.
This is the single highest-leverage fix for 'my page looks amateur'. A full-bleed paragraph on a 27 inch monitor is 200 characters wide and physically tiring to read. CSS has a unit for exactly this: 'ch' (width of the zero glyph), so 'max-width: 65ch' is a one-line cure. The newer 'max-width: min(65ch, 100%)' keeps it from overflowing small screens.
Gotcha: measure is tied to leading. Longer lines need more line-height to stay trackable, shorter lines need less. Bumping a column from 45ch to 75ch without loosening line-height gives you a gray slab. Also note 'ch' is font-relative, so a wide font makes 65ch physically wider than a condensed one at the same value.
Ask AI for it
Constrain the body copy in this article layout to a comfortable measure: max-width: min(65ch, 100%) on paragraphs and lists, centered with margin-inline: auto. Let images, code blocks, and pull quotes break out wider than the text column. Pair it with line-height 1.6 for body and 1.2 for headings, and keep captions at a shorter measure of about 45ch.