Content width
How wide the reading column is allowed to get. Roughly 60 to 75 characters a line before the eye starts losing its place.
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What it is
Content width is how wide the reading column is allowed to get. Typographers call it the measure, and they count it in characters rather than pixels: roughly 45 to 75 characters per line, with about 65 as the comfortable middle. Too wide and the eye loses its place jumping back to the start of the next line; too narrow and it breaks the rhythm with a return every few words.
The move most pages need is two different widths: a wide shell for layout (grids, cards, images, hero) and a narrower column for prose inside it. Around 640 to 720px of body text under a 1200px container is the standard editorial setup. CSS gives you the unit directly: 'max-width: 65ch' on paragraphs measures in the actual typeface instead of guessing.
Gotcha: 'ch' is the width of the digit zero in the current font, so the same 65ch is visibly wider in a monospace font than in a condensed sans. Check it, don't assume. Also, content width is not one number for the page: captions, pull quotes, and code blocks are allowed to break out wider, and code in particular hates being wrapped at 65 characters.
Ask AI for it
Constrain the reading width of this article: set the prose column to 'max-width: 68ch' (about 700px), centered, inside the existing wider page container. Let images, code blocks, and pull quotes break out to the full container width while paragraphs, headings, and lists stay in the narrow measure. Keep 16px of side padding on mobile and don't let the column exceed 75 characters per line at any viewport size.