Design Nomenclature / Type
Measure
Also called line length, column width
This entry has not been translated yet and is shown in English.
The horizontal length of a line of text, historically in picas and now most honestly in characters. Somewhere around 45–75 characters is the workable band, with 55–65 the centre of it. Too long and the return sweep loses the next line; too short and the eye is thrown back before it builds momentum.
In practice
CSS gives you the ch unit, so max-width: 65ch expresses the rule directly and tracks the face. Measure and leading are one decision, not two — set them together.