No matter their training or specialism, designers are united by a love of letters. Typography is often described as an art or treated as a means of expression, but at its most fundamental level, it’s a powerful tool. Whether the medium is print or digital, typography plays the most important role. It ensures a product is functional, creates hierarchy and determines legibility. Alongside all this, it makes an impact on an emotional level, conveying a sense of a brand’s character.

Type is beautiful — so far, so good… print and digital designers are all on the same page (as it were). There’s a clear line where the standards of perfection divide, however. I’m talking about line-height, or leading.

For clarity about some of the terminology to follow in this article, check out my ‘anatomy of type’ glossary. If you’re already down with the lingo, read on…

When it comes to print, the standards for legible leading are much tighter. The most long-established starting point is to ‘add two’. So if your font size is 10pt, this makes a line-height of 12pt a good fit. There are a few ways of expressing this principle and some textbooks talk about 1.2 or 120% — but the idea is the same. We’ll call this the ‘traditional model’:

“For a 10pt font the usual leading is 12pt, creating a 10/12 ratio, which is considered single spacing.” Elements of Typographic Style (2012)

For interface designers, bigger is better: if legibility is important, choose a tall line-height. Authorities in the UI community guide type practitioners towards a ratio of 1.5 or 150%. Let’s call this the ‘digital model’:

‘Once you have decided on your body copy font size, multiplying this value by 1.5 will give you the optimal line height.’ Smashing Magazine (2009)

To test the influence of these lessons, I printed a single page with two paragraphs of the same text, one set looser than the other. I showed this to designer friends from different backgrounds and asked them which was easier to read. The print designers unanimously preferred the tighter version, while most of my colleagues in UI felt strongly about the opposite.

So what’s going on? Have standards of functionality really changed, or has there been a shift in the perception of beauty? What are today’s rules, and whose authority should we listen to?

Many UX designers reference the W3C Accessibility guidelines as proof that looser line-height is more legible. Here the recommendations state that:

Visual Presentation: For the visual presentation of blocks of text, a mechanism is available to achieve the following: (Level AAA)

Foreground and background colors can be selected by the user.

Width is no more than 80 characters or glyphs (40 if CJK).

Width is no more than 80 characters or glyphs (40 if CJK). Text is not justified (aligned to both the left and the right margins).

Line spacing (leading) is at least space-and-a-half within paragraphs, and paragraph spacing is at least 1.5 times larger than the line spacing.

Text can be resized without assistive technology up to 200 percent in a way that does not require the user to scroll horizontally to read a line of text on a full-screen window.

You can’t argue with the importance of accessibility and legibility… of course interfaces need to be inclusive. But it’s interesting that these guidelines appear to ignore the distinct anatomy of different typefaces. In print, the x-height of a typeface would typically affect line-height and would, therefore, be optically optimized. Why should the rules be different for UI design?

I started analyzing the typography of popular interfaces that focus on legibility to learn more, and to see if designers are actually following the W3C advice.