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Law of legibility
Text characters must have sufficient size, style and contrast so that they can be read with minimal effort.
Contrast, size and leading come before the choice of typeface.
Legibility is not a matter of taste and is not judged by the eye of whoever drew it. It depends on size, contrast and spacing, and all three have values that can be measured. That matters because the designer is almost always the person in the best reading conditions of the whole chain: good screen, well-lit room, rested eyes, and already knowing what is written. Whoever is going to use it may be in the sun, on an old device, in a hurry or forty-five years old. The text that just scrapes by for you does not pass for that person.
What the brain does with it
The biological mechanism behind the effect, and how much of it was actually measured.
V1The first area of the brain, right at the back of the head, to receive what the eye sent. It is where the image becomes contrast, edge and orientation, before you recognise what you are looking at.See it in Senses and attention →See in the glossary → separates the stimulus by orientation and contrast, and a region of the left ventral temporal cortex responds selectively to written words: that is the stage where stroke becomes symbol. Small type, low contrast or insufficient spacing make that passage more expensive, and the cost shows up in reading time and in error rate. That is where it should be measured. Retention has already been tried as a measure of legibility by the disfluency literature, from 2011 on, and the replications came back null. Stress, nobody has ever collected in a study of interface legibility.
The body this law presumes
Who it was measured on, and what changes when the body on the other side is another.
Low vision, dyslexia and presbyopia change the legible minimum, and what replaces the designer’s choice is the person’s control over size and spacing. The environment comes in with it, because the contrast that passes on your calibrated screen vanishes in the two o’clock afternoon sun. And what counts as legible changes with the script: spacing, x-height and weight do not behave the same in Latin, in Arabic and in ideographic script. As for the typeface “for dyslexics”, the test exists and came back negative. KusterCompared the Dyslexie font with Arial and Times in children with and without dyslexia, and found no gain at all. The children preferred the others.One reference in this work:2018Dyslexie font does not benefit reading in children with or without dyslexiaSee in the bibliography → and colleagues compared Dyslexie with Arial and Times in children with and without dyslexia and found no gain in speed or in accuracy, the children even preferred the others. What does show an effect is increasing the spacing between letters, which is not a special typeface, but an adjustment any text accepts.
How design translates it
What to do with it on a screen, without turning a finding into a rule.
How to measure this in your product
Contrast, size, family and spacing between lines and letters, always thought out for the reading context: the device, the ambient light, the age and the eye conditions of whoever reads. The useful question in review is who is in the worst condition to read this, and the answer is almost never you, who drew it. On screen, the choice of family yields less than it promises, because what changes performance is size, contrast, spacing and familiarity. Before choosing the typeface, give the person control over size and spacing.
Where it breaks
Where the law does not hold, holds less, or holds in reverse.
Legible is not the same as understandable. A text with perfect contrast and the wrong vocabulary passes any automated audit and still goes without being understood.
Related neuromyths
What gets said about this concept out there, and what the evidence actually shows.
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We read by the word’s silhouette, which is why ALL CAPS is illegible.
The word-shape model has been abandoned: the evidence supports parallel recognition of letters, and the region that responds to written words is invariant to case and typeface. All caps is read more slowly, but out of familiarity, not silhouette.
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Typefaces for dyslexics, such as OpenDyslexic, help the dyslexic brain read.
Controlled tests find no advantage of these typefaces over well-spaced ordinary type. What actually helps is greater spacing between letters and lines, which any typography allows, and which serves everyone.
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Sans-serif is better on screen; serifs guide the eye on paper.
The literature is null: a review of 72 studies and half a century of research found no reliable difference in legibility between the two. What changes performance is size, contrast, spacing and familiarity.
Um caso
A real product where this showed up, with what happened and where to check it.
The more legible typeface the road returned
Foto de MPD01605 · via Wikimedia Commons · Domínio público
In 2004 the United States federal highway agency approved the Clearview typeface for dark-background signs, based on early studies that showed it to be more legible. In 2016 the same agency ended the approval. Later evaluations found no benefit in the narrower letters and found legibility worse than that of the standard alphabet, above all on light backgrounds. The explanation is on the agency’s questions page. The gains measured at the start came from the signs’ new reflective material, not from the lettering. In 2018 Congress ordered the approval reinstated, and the agency recorded that it neither recommends nor endorses the style. The case is not about screens, and it serves for that very reason, because there the variables were kept separate. Swapping only the retroreflective sheeting already yielded a 6% gain in legibility for the standard alphabet itself. The change of style on its own yielded no practical difference on dark backgrounds, and got worse on light backgrounds and in the numerals. Before choosing your product’s typeface, sort out the contrast.
Experimente
A piece to check in your own body what the text has just claimed.
Contrast is not judged by eye
Change the size and the clarity of the text. The number that appears below is the contrast ratio, calculated with the WCAG formula — the international web accessibility standard. It says whether the text passes or fails.
The retina takes in far more than the brain processes at once. Legibility is how much of that input becomes effortless reading, and contrast is the part that can be measured.
- Contraste
- 1.05:1
- WCAG AA (4,5:1)
- ✕ reprova — falta 3.45
The marks are normative: at AA, 4.5:1 for normal text and 3:1 for large text; at AAA, 7:1 for normal text and 4.5:1 for large text. The scale goes up to 21:1, which is pure black on pure white.
Contrast is not judged by eye. The WCAG floor is 4.5:1 for normal text, and 3:1 for large text. Notice that increasing the size changes the applicable criterion — the same colour can fail in a paragraph and pass in a heading.
Fontes
Enunciado in the field’s current form. Dyson, 2004, measured the physical configuration of text on screen, line length, columns and leading, and not size or contrast.
- DYSON, M. C. How physical text layout affects reading from screen. Behaviour & Information Technology, v. 23, n. 6, p. 377-393, 2004. DOI
Leitura complementar: Legibility, Readability, and Comprehension NN/g em inglês