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.
