Join William Lidwell for an in-depth discussion in this video Legibility, part of Universal Principles of Design.
- [Voiceover] Hi, I'm Jill Butler and this is the Universal Principles of Design. In this movie, legibility, or why you should beware the fine print. At 4:45 PM on May 26, 1987, Air New Orleans flight 962 took off from New Orleans International Airport. There were two pilots and nine passengers on board traveling to Egland Air Force base in Florida. As the British Aerospace Jetstream 31 reached an altitude of about 200 feet, the pilot noticed that engine gauges were fluctuating erratically and proceeded to attempt an emergency landing.
The plane touched down then crashed through an airport security fence, damaged a concrete barrier and rolled across the Route 61 Highway. Several cars were hit by the plane before the main wreckage came to rest in a parking lot. Two passengers aboard the plan suffered serious injuries. Both pilots, seven passengers and two people in vehicles on the highway suffered minor injuries. The airplane was destroyed.
It turns out that the crew had failed to advance the engine levers to the high RPM position before takeoff even though this was the last item on the official flight crew pre-flight checklist. So why did the pilots fail to complete this critical step? In the accident report, the Natural Transportation Safety Board noted, the typeface on the Air New Orleans checklist is 57% smaller than that recommended by human engineering criteria. This smaller typeface reduces the legibility of print even under optimum conditions.
In other words, legibility or rather illegibility, may well have been the root cause that crashed the plane. The term legibility refers to the visual clarity of text, generally based on size, typeface, contrast, case, line length and spacing. Legible text is text that is clear and easy to read. To ensure text legibility, follow these guidelines. Size.
In print documents and presentation software, type is measured in points from the top of the ascenders to the bottom of the descenders. For printed, large blocks of text, nine to twelve point type is the most legible. Smaller sizes are acceptable when limited to small chunks of text like image captions and footnotes. Larger type is better for low-resolution digital displays and for younger and older audiences. Typeface.
Despite what you may have heard, there is no performance difference between sans serif and serif typefaces, so make your selection based on aesthetic preference. Professionally designed typefaces are the safest, most legible choices. Contrast. High contrast between text and its background increases legibility. Use dark text on a light background or light text on a dark background. Color does not affect legibility but contrast between colors does.
Background patterns and textures should be avoided as they can dramatically reduce legibility. Case. Sentence case should be used for text blocks. Text set in all caps takes more time for readers to process and should be reserved for small chunks of text, like headings and sub-headings. Line length. In large text blocks, the length of each line of text should be 10-12 words per line, which equals about 35-55 characters per line.
Spacing. Leading is the amount of vertical pace between lines of text and is measured from baseline to baseline. Leading should be equal to the chosen type size plus one to four points. Consider legibility anytime you are working with text, especially in educational, advertising and communication contexts. Improving legibility in your documents makes them more effective and better at communicating their message.
So whether you use your knowledge of legibility to typeset a best-selling novel, to ensure that written warnings are visually clear in software or to improve flight checklists and reduce airline accidents, remember, always beware the fine print.
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