What Is X-Height, and Why Does It Decide Legibility?
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Set Garamond and Helvetica side by side at 10 point. Both are 10 point. Only one of them looks it.
X-height is the height of a typeface's lowercase x, measured from the baseline to the top of the letter. X-height decides how large a typeface appears at a given point size, and it is the strongest single predictor of whether text holds up once it gets small. Point size measures the box. X-height measures the letters.
What does x-height actually measure?
X-height is the distance from the baseline to the top of a lowercase x. The baseline is the invisible line the letters sit on. Cap height is the distance from that same baseline to the top of a capital H. Ascenders are the strokes that rise above the x-height on letters like b, d, and k. Descenders are the strokes that drop below the baseline on p, q, and y.
Point size measures none of those. Point size measures the em, the invisible square a typeface is drawn inside. Where a designer places the x-height inside that square is a decision, and different designers made different ones.
The useful number is therefore not x-height on its own. It is x-height as a proportion of cap height. Set a face at 100 point, measure the lowercase x, measure the capital H, and divide. A ratio near 0.70 is a tall x-height. A ratio near 0.60 is a short one. The gap sounds trivial written down and is unmistakable on the page.
Why do tall x-heights read better at small sizes?
Typefaces with tall x-heights hold legibility at small sizes because the white space inside each letter stays open longer. That enclosed white is called the counter. The counter is what your eye uses to tell one letter from another, and it is the first thing to close up as type shrinks.
A tall x-height means bigger counters at the same point size. Helvetica, Univers, and Verdana stay readable at 9 point in a way Bodoni does not, and the reason is mostly counter size.
Matthew Carter drew Verdana for Microsoft in 1996 with an unusually tall x-height and generous spacing, for screens with a fraction of the resolution of the one you are reading this on. The screens improved. The typeface kept getting used, because the problem it solved was never only resolution.
Why a short x-height is not a defect
A short x-height buys length for ascenders and descenders. Inside a fixed em, the room has to come from somewhere. Lower the x-height and the b, d, k, p, and y get space to extend.
Long ascenders and descenders give words distinctive silhouettes, and silhouettes are how you read at speed once type is large enough to read at all. Garamond carries four hundred pages. Helvetica does not, and nobody sets a novel in it by accident.
The rule is not that taller is better. Taller is better small. Longer extenders are better long.
The sizing mistake the spec will not catch
Specs are written in point sizes, and point size does not describe how big the letters are. A brand guideline that sets body copy at 10 on 15 is describing a box, not a page. Change the typeface, honor the spec exactly, and the page changes size.
In our type classes at FIT, the version that turns up every semester is a junior designer who picks a face with a short x-height for a caption, sets it at the size the grid calls for, and then cannot say why the caption feels thin. Nothing is wrong with the typeface. Nothing is wrong with the number. The number was written for a different x-height.
The fix is not a bigger number. The fix is deciding which of the two you are specifying: a size, or an apparent size.
Try this today
Set the same lowercase paragraph twice, in two faces, at identical point size, measure, and leading. Not a headline. Body copy, fifty words minimum, because x-height only shows itself in a run of lowercase. Take both down to 9 point and back away from the screen. Whichever one is still a paragraph rather than a gray texture has the taller x-height, and now you know it by eye instead of by spec sheet.
Point size is a container. X-height is a decision somebody already made about how to fill it, which means choosing a typeface is choosing an apparent size before you have typed a number.
Type is the material. Knowing what the material does when it gets small is the difference between specifying and guessing.
FAQ
Is a bigger x-height always better? No. A tall x-height wins at small sizes and on screen. A short x-height wins in long-form print, where ascender and descender length gives words recognizable shapes.
How do you measure x-height? Set the typeface at 100 point. Measure the lowercase x from baseline to top, then measure the capital H. Divide the first by the second. That ratio is comparable across typefaces in a way a raw measurement is not.
Does x-height affect font size? X-height does not change the point size. X-height changes how large the point size looks. Two typefaces set at 10 point can differ by roughly twenty percent in apparent size.
C.J. Yeh is a founder and ECD at Cynda Media Lab and a co-founder of DSGN BY D. He is also a tenured professor and curriculum chair of Creative Technology and Design at the Fashion Institute of Technology.
Sources
- Microsoft Typography, Verdana release documentation, 1996.
- Matthew Carter, on the design brief for Verdana.