It barely matters how many books on typography I’ve read and will read: I struggle just as much as anyone else when it comes to finding a good typeface for a particular project. The message, the mood and the context, they all play equal parts in the decision in finding the right typeface for the job.

There is however, an often overlooked aspect that I’d like to make a good case for the next time you’re hunting for the one typeface, and that is to learn how to assess its x-height impact on readability and legibility. Over the years I’ve been learning how to pay more and more importance to it, and here’s why you’ll maybe want to understand it better a bit too.

What is x-height?

The x-height is literally what it means: the height of a lowercase x character for a particular typeface, measured from its baseline. This height should be seen as a relative unit for measuring the proportion of the lowercase letters: the height measurement alone, as a unit, doesn’t tell us much.

x-height for the Adelle typeface

Why does it matter?

You may have or may have not noticed that x-height varies a lot amongst typefaces (example a couple of paragraphs below). Now consider that you are reading this piece of text in lowercase letters: this paragraph isn’t a street or a highway sign to be read in 0.2 seconds, but a continuous flow of words which were carefully put together on a medium that respects this context. Different contexts or purposes call for different readability and legibility choices, and x-height has a bigger impact on both of these than, perhaps, the choice between going serif or sans-serif.

So, large or small x-height?

Here’s the interesting (though often frustrating) bit: you’ll find references among different knowledge sources stating that sometimes, a larger x-height is a better option for a legible piece of text, and others, a smaller one is the best. So where do we stand? The first thing to note is that yes, a larger x-height certainly gives the appearance of having better legibility. But that’s not the entire story!

Let’s look at two popular sans-serif typefaces, Gill Sans and Fira Sans, put together: