When reading something continuously, it is nice to have the text width of the page rather short, so that the reader’s eyes don’t have to travel too far to find the next line.
When scanning a page, looking for information, it is nice to have the text width wide so that more content fits on the page. This allows the eyes to scan more content looking for the spot to begin reading. Otherwise the reader has to scroll the page while also scanning with the eyes.
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