Measure

The word is letterpress. A compositor set the composing stick to a width, in picas, and that width was the measure; every line in the job was built to it. What carried over to the screen is the habit of stating it in characters rather than in picas or pixels, because the constraint is about the eye and not about the paper. At the end of every line the eye has to sweep back and drop one line, and the longer the sweep, the more often it lands on the line it just read.

The numbers everyone quotes come from Bringhurst: anything from 45 to 75 characters is satisfactory for a single-column page, and 66 characters, counting letters and spaces, is the figure he calls ideal. Treat that as a well-chosen convention rather than a finding. The research underneath is messier and mostly says two things at once: readers often move through longer lines slightly faster, and they reliably prefer and report better comprehension with shorter ones. When speed and preference disagree, interface text should follow preference, since nobody is being timed.

In CSS the unit that matches the concept is ch, the advance width of the zero glyph, so max-width: 66ch is the direct translation. It runs a little wide in practice, because in most faces a zero is wider than an average lowercase letter and much wider than a space, which means a 66ch column holds closer to 70 or 75 characters. Setting somewhere around 60ch usually lands on Bringhurst’s 66 in real text. The newer lh and rlh units make the companion adjustment easy to state, since a long measure needs more leading to keep the return sweep from missing its target, and a short one needs less.

Measure is the setting a full-width layout forgets. A paragraph in a div with no constraint inherits the viewport, which on a wide monitor is 200 characters and unreadable, and the fix is a maximum on the text column rather than a maximum on the page. The other failure is a measure so narrow that every line breaks after four or five words: the rag turns violent, hyphenation starts working overtime, and the paragraph reads as a list.