Legibility is not a helpful idea.

The word legibility causes confusion among designers and clients because it is ill-defined. It is used in informal speech to mean a lot of different things, and often this can confuse rather than clarify.

Legibility is often used more as a moral category, or to frame an insult – “this is illegible!” – than with any real precision.

Legibility sounds like it ought to be a real thing, but try to define it in any useful way and it becomes clear just how slippery it is.

For example – is something legible when people can read it quickly? Well maybe, but maybe reading something quickly takes more mental effort.

How do we know how much mental effort goes into something when mental effort cannot be measured? We don’t know. So is reading speed a good measure of legibility? Maybe, maybe not. And what does reading speed really reflect?

Alright, you may say, what about individual letters? Can they be more or less legible? If someone recognises one style of e more quickly than another, is this not more legible? If I can tell a 1 from an I then that is more legible, right?

What is legibility anyway?

The ability to recognise letters and tell them apart rests on two things that need careful separation.

Firstly, there is the physics of sight, the ability of our eyes to resolve detail.

If you can tell an e from an o, what lets that happen in the first place is the eye resolving the details of the letter. How light enters the eye, falls on the retina, how physical sensations are made conscious, is half physics and half perception. It is realm of psychophysics.

Letters may have small details that are more or less easy for the eye make out. But if the letters are big enough to be seen then the small details will be big enough to be seen and the letters will be read. They will be “legible”.

The only lesson that can be drawn from this is that letters with fine details should not be used where they cannot be made big enough for any of their fine details to be seen clearly. Legibility in this sense depends on how big you can make the letters.

Some styles of letters, without fiddly details, may be easier to make out if you have to fit something in a fixed space at a fixed distance. But this is a relationship between the type and its size, not anything inherent within the typeface.

The real question here is – why is your type borderline invisible in the first place? And the solution is obvious – do not to use type that is too small.

Recognising letters is reading

Then there is the learned ability of people to recognise a letter.

Recognising letters means comparing the shape you are looking at in the real world, on screen or in print, to the one in your head, the letter you are expecting to see.

What you are expecting to see is what you have learned. It is made up of all the impressions you have ever had of what that letter looks like.

The ability of people to recognise letters is a learned process. It is the shared cultural experience of reading. We have to learn to read, to associate arbitrary shapes with sounds and meanings.

What is obvious but seldom dwelt upon is that we also have to learn that these arbitrary shapes come in lots of different styles – typefaces. There is no one correct version of each letter that all other variations are derived from, just variations of an arbitrary, socially agreed symbol. What kind of e are people thinking of when they think of an e?

Posing the question in this way reveals how vague the idea of legibility is. If people are recognising letters quickly, what is their basis of comparison? They are comparing a letter to every other instance of that letter they have encountered, learned from and internalised. There is no one, gold-standard, super-legible letter shape against which all others can be measured.

Is there anything more useful than legibility?

Is there a better word than legibility to think and talk about how easy or difficult something is to read?

A clearer word and more measurable property of letters is discriminability. How many similar features does one letter have compared to another, and how many different? “Use discriminable elements” is a piece of advice given by ergonomists and even NASA.

Letters which are easily told apart may be useful when they will be seen in isolation and you have no other clues to recognise them. But to call this feature of letters “legibility” is not exact enough, particularly when discriminability is a more precise term. And making letters distinct from one another can make the letters stop looking like themselves. There’s only so much you can do to an l before it stops looking like an l.

Legibility as a displacement activity

Legibility is often a distraction from bigger design problems.

Signs – my area of expertise – are very often neither big enough nor bright enough. The style of letters cannot compensate for these failings. Becoming preoccupied with the details of one typeface over another steals time from the necessity of arguing for bigger, brighter signs.

Why bother?

The reason to separate the physical side of reading from the cultural side is it helps define the answers to practical problems more clearly. To solve a problem you have to be able to put it in the right category.

If people can’t read something is it because they can’t see it, or because they don’t understand it?

If they can’t see it, it is a physical problem – the text is not big enough or bright enough to be made out.

If they can’t understand it it is it a cultural problem. The letters are so stylised and weird-looking that they’ve stopped looking like any script known to humanity.

In conclusion

Legibility needs to be put into context, and taken with a pinch of salt. Most problems with reading type can be fixed with type size and illumination.

Where letters need to be told apart, discriminability is a useful idea.

Most of all, beware the unstated claims behind legibility talk, that some types are morally better than others.