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Legibility is a fundamental aspect of graphic design and typography. Unlike readability, though which is the characteristic of the overall text, legibility refers to the clarity of single characters of type. While the two are distinct, they are related and should be considered together when selecting a typeface for a particular role or function. Legibility describes a font, readability its function.

Weight – be that thin, regular of bold all have a role to play. A typeface of medium weight are the most readable. Where a typeface is too light it becomes difficult to distinguished from the background. A typeface that is too bold can lose the counterform (the negative space that is fully or partially enclosed by the shape of each letter) that is essential to good legibility. Legibility is reduced in fonts that are designed with either extremely thick or extremely thin strokes. This is because thin strokes are less visible and counterforms are decreased by the thick strokes. Fonts with greatly contrasting strokes will also, when used in long passages of text, generate a visual momentum or dazzle effect, making reading difficult.

Serif / San serif – there is no clearly definitive answer when discussing legibility relating to a typeface with a serif or without. Historically serif type has be selected for long passages of text, dominating book production. Contemporary studies find little real difference in legibility between serif and san serif typefaces! When selecting typefaces, take into account not only their legibility and how readable they will be in their specific application, but also the reading audience and the document’s final destination (screen / print etc).

Type size – is one of the most important but controversial elements when discussing legibility. It is an obvious fact that when text that is too small or too large it impacts upon reading – small type reduces visibility and large type forces the reader to view the document in sections rather than as a whole.

It is common to access type set in print across a range of sizes from between 8 ½  to 15 point. All of these sizes can be comfortable to read, when set correctly. These are physical, printed pages, held in the hand of the reader and viewed within arm’s length. Therefore, using the point system as the form of measurement is the correct method of describing them (72 points equal one inch). But, some still describe type size in ‘points’ when discussing ‘digital design’ which is incorrect. Digital design is viewed on a computer monitor, tablet or smartphone, and all screens are calibrated and measured in pixels not points.

Apple computer screens were calibrated to 72 dpi (dots per inch) or ppi (points per inch) relating to the original point system, with other brands of computer manufacturing their screens to 96 dpi. Now with LCD / LED developments in computer monitors even Apple has changed, there is no standard calibrated size. As there is no clear bench mark, impacting on how to describe, qualify or discuss legibility in terms of digital size. Therefore as a rule of thumb, 96 dpi has to be used as a bench mark.

X-height – defines the relationship between the body and the ascenders. Typefaces set in the same size may look larger or smaller depending on their respective x-heights. So, typefaces with larger x-heights can be set in smaller sizes, usually without impacting on both legibility and readability.

Column or line length – Lengthy passages of text are easier to read if they are kept to a short line length – from between nine and thirteen words. For greater readability in long, thin columns, select a typeface with a smaller x-height. Optimum word length in short columns is between four and five words, no less.

Justified / unjustified – type that is set flush left (also known as ragged right) often has more even spacing throughout each line of type. Because the type is aligned on the left, and because each line ending at the right is either longer or shorter than the next, readers can easily locate the beginning of a new line.

Take care not to rag the type too much when you set ragged right text. The extreme ragged edge can create an uncomfortable rhythm and draws attention to the shape of the column (not its contents). A ‘soft’ ragged edge should look like a piece of paper loosely torn from top to bottom. Yet, fine typographic detailing should consider a ‘saw tooth’ edge – alternating from short to long lines of text.

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Further reference:

Royal National Institute of Blind People: Accessible Information – clear print guidelines.

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Royal National Institute of Blind People: Accessible Information – guide for business

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Sensory Trust: Making information accessible: Clear and Large Print

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UK Government: Accessible Communication Formats

  • Government guidelines – the different contexts for publishing information.
  • A small amount of information in relation to type size, choice and application.
  • Little regarding design and layout,

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