Have you ever noticed that some textbooks seem easier to follow than others? This is usually because of the way they are laid out.
The eye is a muscle and, just like your arms and legs, it gets tired. If a line of text is overly long, the eye tires more quickly. This is because the eye doesn't track a line of text in a straight line; it tackles the data in spans of about six characters and spaces, leaping from span to span in sections.
The longer a line, the more tired the eyes' rod and cone receptors become. They desperately need a microsecond's pause to recharge. And after every so many lines a slightly longer pause is needed. And after a few pauses an even more extended pause is required.
These pauses are provided naturally by line endings, paragraph breaks, and page turnings. Research shows that the optimum line length in English is between 30 and 65 characters (6-11 words). The optimum paragraph is about seven lines, and the ideal page setting extends to no more than 32 lines, though publishers sometimes push this up to 38 lines to save paper.
Type should be between 10 and 12 pt (there are 72 pts to an inch so 11pt is just under 1/6"), otherwise it will be hard to read. Anything smaller is difficult to read and impossible for the 4 percent of readers with impaired vision (American Foundation for the Blind - Survey 1994-5).
If the space between the lines of type is too tight, the eye has to work harder to avoid mixing up the lines (remember it arcs). If the lines are spaced too far apart, it becomes tiresome to maintain a regular reading rhythm. Aim for the following:
♦ 65 characters per line
♦ Spacing between lines M-2pt
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