Over a hundred years of research have shown a correlation between the length of a line and reading speed, and indicated a “sweet spot,” or optimum length of a line that allows for a quick and enjoyable reading experience. The continued presence of this sweet spot—lines that are around 4 inches long—is why physical newspapers still present information in columns.

Julie C. Meloni, HTML, CSS and JavaScript All in One, Sams Teach Yourself: Covering HTML5, CSS3, and jQuery