Understanding Legibility
By: Mert Tol
Tags: UX, Fonts, Typography, UI, Design

Legibility and readability are not the same. Legibility certainly influences readability and vice versa, but to understand how one influences the other it is necessary to consider them separately.

The degree to which a typeface is legible is entirely dependent on the designer of the typeface, whereas readability is largely the province of the typographer. Legibility is the degree to which individual letters can be distinguished from each other. Such letterforms are designed to present themselves clearly and concisely. This does not necessarily mean that a highly legible typeface cannot also have distinguishing characteristics — some of the most legible examples, such as Johnston’s font for the London Underground, Underground, are also among our most distinctive faces — but it does mean that in the most demanding of environments, their forms remain highly visible, essential for those passengers for whom the station names are unfamiliar.

Johnston’s Font for the London Underground

Generally, the most legible typefaces are those with larger, open or closed inner spaces. This inevitably means a generous x-height.

However, if the x-height is large, then, as a consequence, the ascenders and descenders will be relatively short.

This not only affects the legibility of individual characters (commonly causing, among other pairs, the h and n, and the i and l to be confused with each other) but also makes the recognition of word shapes more difficult.

Large counters (the enclosed and partly enclosed spaces within letters) are particularly important in helping to distinguish between some of the most commonly used characters — e, a, and s; and c and o. These lack distinguishing characteristics (they have no ascenders or descenders, and are all of the similar width and general shape) and contain similarly sized counters.

There is little doubt that owing to the frequency of use, the most helpful aid to legibility in any given typeface is the provision of a generous “eye” for the e and enclosed a generous counter for the a.

The characters most commonly mistaken for each other are i, j, and l; and f and t. In many typefaces, the l is also almost identical to the numeral 1, and the letter O to the zero 0. Relative legibility is also affected by the individual size of letters; for instance, m and w are intrinsically more legible than i or l simply because they have a larger presence.

Having considered legibility in terms of distinctive word shapes and definition of structural elements of letters (ascenders, descenders, counters, and serifs), there is a third aspect — that of type size. Using a small size of type, perhaps 6pt or less will deny the text to a large proportion of the targeted audience. We know, when a small size is unavoidable, then its deficiencies can be minimized by the use of a typeface with a large x-height.

Finally, another condition which governs legibility is tonal contrast, for example, between word and substrate. Naturally, where word and background tone is close, legibility will be affected.

What is less obvious is that a text printed black will be more legible on a matte, off-white (cream) coloured paper than a gloss high white. The mix of surface shine and high contrast can prove both irksome and tiring.