What Is x-height?

Look at a line of lowercase text and you will see three zones. Most letters sit in a central band: a c e m n o r s u v w x z. A few reach above it (b d f h k l t), and a few drop below (g j p q y). The x-height is the height of that central band, measured from the baseline up. It is named after the x because that letter has flat ends and no overshoot, so its height is easy to measure.

Why two fonts at the same size look different

Point size measures the em square, an invisible box, and not any visible letter. Two typefaces set at 16 points can differ a lot in how big their lowercase looks, because their x-heights differ. Georgia, designed for screens, has a taller x-height than Garamond, so Georgia looks larger and darker at the same nominal size. This is why swapping a fallback font on a website can make text appear to jump in size.

The ratio of the x-height to the cap-height matters too. A low ratio gives tall, graceful capitals over small lowercase letters. A high ratio, as in many grotesque sans-serifs, makes capitals and lowercase look closer in size and gives an even, modern colour.

x-height and readability of small text

At small sizes the central band does most of the work: the eye reads the shapes of the letters in that band, with ascenders and descenders adding clues. A taller x-height has three effects:

1. The lowercase letters occupy more of the available size, so they are easier to see.
2. Counters (the enclosed or partly enclosed spaces in a, e, g) stay larger and do not fill in.
3. The space between lines shrinks relative to the letters, so leading usually needs to increase.

This is why screen-first fonts and caption fonts have generous x-heights. It is also why styled Unicode letters in tiny bios can be hard to read: script and Fraktur letters often have small central bodies with large loops, and a small caps style raises the apparent body size back up by using capital shapes at lowercase height.

x-height in hand lettering and handwriting

Lettering starts with guidelines, and the x-height line is the one that most changes the character of the alphabet. Typical teaching schemes use three or four lines: baseline, x-height (also called waistline), cap-height and the ascender line. Traditional copperplate-style scripts keep a small x-height with tall ascender and descender loops, while a modern, bouncy script sets the x-height larger and shortens the loops. If you move only that one line, the same letter shapes feel completely different. You can see the zones on each of our cursive letter pages and in how to write in cursive.

x-height and fancy Unicode text

Each style from a generator is drawn by whichever font the device picks, and fonts differ in x-height. Math Alphanumeric characters such as 𝐛𝐨𝐥𝐝 and 𝑖𝑡𝑎𝑙𝑖𝑐 are designed so that they match in height with each other within a font, but when they fall back to a different font than your ordinary text, the result can look bigger or smaller than the surrounding words. Mixed text in a post can therefore look uneven even when nothing is wrong with the characters.

Common confusions

• x-height is not font size. It is a proportion of the design, not a size.
• x-height is not cap-height. One measures lowercase, the other capitals.
• x-height is not the height of every lowercase letter. Ascenders and descenders go beyond it, and round letters slightly overshoot it.