Measure and leading, and why 66 characters is a starting point rather than a rule

Filed under measure

The advice you meet first is that a line of text should run to roughly sixty-six characters, spaces included. It is decent advice. It is also repeated as though it were a constant of nature, which it is not — it is one point on a curve, and the curve has a second axis nobody mentions.

The two knobs move together

What actually goes wrong on a long line is the return sweep: your eye reaches the right edge, travels back left, and has to land on the start of the next line rather than the one above or below it. The longer the journey, the more room the landing needs. So measure and leading are not two independent decisions. Widen the column and you must open the leading to match, or the reader starts re-reading lines and blaming themselves for it.

A column at 45 characters is comfortable at fairly tight leading. The same type at 85 characters is not comfortable at any leading, but it is markedly less bad at generous leading than at tight. Treat the pair as one setting.

Long lines are not automatically worse. Long lines set at short-line leading are worse, and that is the combination people actually ship.

Characters are a proxy, and a leaky one

Counting characters is convenient because ch units exist and because you can eyeball it. But the eye is measuring physical width, and the same character count is a different width in a condensed face than in a wide one, in a language with long compound words than in one without. If you set a German page to the same character count as its English original, the German column comes out visibly wider.

The character count is a starting point that saves you from starting at zero. It is not the thing being optimised.

You do not control the window

On paper the measure is decided once. On a screen the reader decides it, by dragging a window edge. All you can really do is set an upper bound and accept whatever happens below it.

This is why a max-width in a typographic unit tends to survive contact with reality better than a fixed pixel width: when the reader bumps their base font size, the column grows with it and the character count stays roughly where you put it. A pixel width holds the column still and lets the count collapse.

Where the rule genuinely bites

Two places. Full-width text on a large display, where nothing stops the line and it runs to 140 characters — this is the common failure and it is entirely self-inflicted. And narrow columns with justified text, where the line is too short to absorb the spacing adjustments and you get rivers of white running down the page. Below about 40 characters, justification stops being worth the trouble.