Line length is the number of characters in one displayed line of text. It is a small typography detail with a big job: lines that run too long are hard to scan, while very short lines make reading feel choppy. section 5.5 section 9.2
For body text on a screen, aim for roughly 50–80 characters per line, with about 65 characters as a useful target. Larger fonts can tolerate longer lines; smaller fonts may need shorter ones. section 9.2 section 5.5
In CSS, control line length by changing the width of the element that contains the text. Width is not a character count, so the resulting line length also changes when the font size changes. No tiny character-count dial, sadly. section 5.5 section 9.2
Narrow a text block
A text block that stretches beyond 150 characters per line is difficult to scan. Giving its containing div a width of 630px produces shorter, more comfortable lines; use border-box when the declared width should include padding and borders. section 9.2 section 9.2
Figure 9.3 In the default width on a large screen, the line lengths of this text are too long for comfortable reading.
Compare the wide default text block with a width-limited version to see why character-heavy lines are harder to scan. section 9.2
