Three Optical Illusions Every Type Designer Has to Fix

At first glance, some letters seem almost too simple. Two lines crossing — a clean X. A perfect circle — the timeless O. But in the world of typography, those “simple” forms are two of the most deceptive shapes you’ll ever draw.

X and O are among the most corrected, adjusted, and discussed letters in type design. Not because they’re complex, but because our eyes don’t see geometry the same way a computer does. What looks mathematically correct rarely looks visually correct — and that’s where the magic of type design happens.

In this post, we’ll break down three optical illusions that every type designer constantly fixes to make typefaces feel balanced, natural, and beautifully human.

The X and the Poggendorff Illusion

The letter X might look like the easiest glyph in the world — two diagonal strokes that meet in the center. Except they don’t.

If you draw those lines so they intersect exactly at the geometric center, the X will look wrong. The top feels lighter, the bottom heavier, and the whole thing slightly off-balance. This happens because of the Poggendorff illusion, a quirk in human perception where diagonals appear misaligned when interrupted or crossed. Our eyes misjudge where those lines “should” meet.

So what do type designers do? They cheat — on purpose.

Designers shift the intersection slightly upward, just a touch, until the X feels optically centered. You don’t notice the adjustment, but you’d instantly feel it if it wasn’t there. That’s why the X is often used as a balance test in type design. If the X looks stable, the rest of the alphabet usually follows suit.

Why the O Is Always a Bit Taller

The O is the ultimate optical illusion in typography.
A perfect circle drawn to match the height of the capital H will always look too small. Round shapes appear smaller than flat ones — our eyes pick up on corners and verticals more strongly than curves.

To fix this, designers use overshoot — extending the O slightly above the cap height and below the baseline. Usually by only a few percent, yet it makes all the difference.
Without overshoot, the O feels undersized and timid. With it, the letter finally aligns visually with the surrounding shapes.

This isn’t just about O, of course. The same principle applies to curved letters like C, G, S, and Q — but the O makes it most obvious. Every good typeface depends on this subtle illusion.

Why the O Is Also Heavier Than It Looks

There’s another illusion hiding inside every O: its curves look thinner than straight lines, even when both have the same stroke thickness.

So if you draw an O to match the vertical stem of an H, it will appear lighter and weaker. That’s why designers intentionally thicken the curves of round letters — to balance the visual weight.

This optical correction isn’t decoration. It’s structural. The heaviest parts of the O aren’t perfectly symmetrical either — the weight subtly shifts as the curve moves around, helping the letter feel balanced and harmonious within the rhythm of a word.

When done right, you never see the adjustment. You just feel that everything looks “right.”

The Bigger Lesson: When Geometry Isn’t Enough

The X and O reveal one of typography’s most fundamental truths: Visual harmony and mathematical precision are not the same thing. Good type design is never about perfect geometry. It’s about how our eyes read shapes in context — next to other letters, across words, and throughout text blocks.

The best adjustments are invisible. They’re what make a typeface feel calm, confident, and effortless. And once you know they exist, you’ll start spotting them everywhere.

You’ll never look at an X or an O the same way again.