Type readability

The ability to read quickly and to understand what is being read depends on good typographic design reducing the effort required to comprehend the text. (It also requires good writing and copyediting, but that’s for another post.)

For readability, the choice of typeface, upper and lower case (U&lc), typesize, leading, line length, margins and paper colour and all need to be combined to produce readable copy.

For instance, most books are set using a serif font rather than sans-serif, because the serifs help the reader’s eye to rapidly scan along the lines of type on a page. This gives U&lc serif faces a high readability factor for longer passages of text.

Whilst sans-serif faces generally have a relatively low readability factor when set to longer line lengths, they have high readability factor on signage and when set in shorter line lengths. Sans-serif offers a sense of urgency, enabling the reader to scan quickly down the copy and assimilate information – a more appropriate reading experience for signs and magazines.

Common text type sizes tend to be between 9 and 12 point (depending on font) with leading set automatically by design software at around 120% of the typesize. This should always be adjusted by the designer with an appropriate amount of leading determined by the design. As a rule of thumb, for normal text, leading should be increased proportionally until around 14 point, when it should be decreased proportionally until around 18 point, after which it should be set solid (ie. 18/18pt). With larger type sizes and headlines, the leading should be further reduced to avoid creating too much space between lines.

In typesetting for legibility, the leading should never be less than the spacing between words, otherwise the readers eye is drawn down rather than across the page. However headline setting can often be set solid, or even to a minus value, without affecting readability. For longer measures, the leading might also be increased so it provides stronger definition to the line of text. As a rule of thumb, the longer the line, the greater the leading required.

In English, the line length normally contains between 50 and 80 characters for readability, resulting in an average of nine to 12 words per line. Scientific and technical papers, which are likely to contain longer words, tend to need a longer measure.

Finally, the margins around the type should allow enough white space to allow the reader’s eye to rest momentarily until it scans back to the start of the next line.

Note that glossy white paper, whilst great for images, makes text difficult to read.

So, so as long as you print on a matt paper stock, readability is governed by the designers ability to structure typeface, case, size, leading, line length and margins into a compelling design.

Type legibility

Legibility is the degree to which individual letters in a typeface can be distinguished from each other. Generally, the most legible fonts have a well-balanced proportion of form (the actual letter) and counterform or counter (the ‘holes’ in the letter). The best designed typefaces tend to have larger counter spaces, and due to their frequency of appearance in text, the most helpful aid to legibility is a generous ‘eye’ for an ‘e’ and an enclosed counter for an ‘a’.

However, for any given font size, too large a counter and corresponding x-height means too short an ascender and descender for clarity between other letters. The characters most commonly mistaken for each other are I, j, l and f and t, so the most legible typefaces need to strike the right balance between the size of the counter and x-height and size of ascender and descender.

Whilst problems of legibility can exist in the design of letterforms in individual fonts (for example lowercase Garamond h and b are easily confused, as are the capital I and lowercase l in Arial), research into typographic design for children by the University of Reading suggests that there is no intrinsic difference between the legibility of serif or sans-serif typefaces. Instead, the context in which they are used assumes greater significance.

Two well-documented experiments on the legibility of type are Emile Jamal’s 1878 demonstration that showed that the top half of a line of upper and lower case (U&lc) type is more legible than the bottom half when only half of the line is exposed, and the 1960’s London Transport tests on all caps or U&lc for bus signage. These series of experiments suggested that whilst there is no definitive proof that lower case is visually superior to all upper case, a mix of upper and lower case was more popular with the public.

This fed into the development of the UK motorway network signage when the Ministry of Transport’s Design Research Unit developed one of the most ambitious information design projects ever undertaken in Britain. In tests on signage designs, sans-serif U&lc type with a wide margin was preferred for legibility.

The result of this work suggests that it is not only the visual recognition of the letterform that is important, but also the shape of the word itself and the space that it sits in that the typographer needs to take into account when designing for legibility.

Legibility and readability

Legible type and readable type seem like synonyms, but typographically they are not the same thing.

Legibility refers to the recognition of individual letters and words, whilst readability is the clarity and speed at which content can be read and understood.

The legibility of a typeface is dependent on its design (and perversely some typefaces are deliberately designed to be illegible), whilst readability depends on the skill of the typographer in choosing a typeface and arranging the type well on the page.

One of the more common mistakes designers make is to choose a typeface designed for one purpose and use it for something else for which it is unsuitable. A classic example is the use of the display face Avant Garde as a text font. Equally, you would not use a difficult to read brush script for a fire exit sign where the meaning needs to be read and decoded quickly.

Since the introduction of the Disability Discrimination Act in the UK in 1995 and the Disability and Equality Act in 2010, typographic design needs to be inclusive of people with a wide range of visual impairments. Making sure that your type is structured to be legible and readable was always a hallmark of good design, but now it is a prerequisite.

In the next two posts I'm going to look at little deeper into some of the issues around the legibility and readability of type.