What it means
A long line can make it harder to find the beginning of the next one, while an extremely narrow column creates frequent breaks and a choppy rhythm. Comfortable measure depends on the typeface, size, line height and kind of content. A heading has different needs from a reading column.
Use it in practice
Read several paragraphs at the intended viewport widths. Set a sensible maximum width for prose while allowing images or other sections more room.
Watch out for
Do not make every block span the full desktop width. Avoid rigid widths that force horizontal scrolling on a phone.
