Here's a pleasant little piece about an element of type design that’s often misunderstood – the ‘O’ – and what makes up its deceptively complex geometry.
The ‘O’ is a simple letter, a simple form. Maybe the simplest form? Draw a circle and… job done? Well, in type design, you’re not quite done yet. In fact, almost no fonts actually use a perfect circle, all ‘o’s are a bit skewed. The O is one of the most deceptively complicated letters in a typeface. It has to balance geometry with perception – an optical feat of feeling like a circle – maintain a consistent rhythm with other letters, and look the same size as shapes that are technically quite different. Its apparent simplicity is precisely what makes it such a useful lesson in how type works.
At its most basic construction, a geometric circle has a constant radius, where every point on its circumference is the same distance from the centre. An ‘O’, however, is usually built from carefully controlled Bézier curves rather than two perfect circles. It is made from two parts, the outer and inner contours, which are adjusted so that the space between them produces a stroke that looks consistent. This is an optical matter, because our eyes don’t perceive the weight of a curved stroke uniformly. Around the sides of the ‘O’, where the curve is more vertical, the letter can appear heavier than it does at the top and bottom. It’s an optical illusion, as horizontal strokes appear thicker than vertical ones. The type designer can compensate by moving the inner or outer contours, changing the width of the stroke in different parts of the letter. The measurements may therefore vary slightly, but the visual weight remains more even. It’s now optically refined.
Another thing type designers consider is the overshoot of an ‘O’. If a circular ‘O’ is placed next to a flat-topped ‘H’ – both with the same cap-height and baseline – the ‘O’ will appear smaller. Our eyes tend to judge flat shapes against their edges, while curved shapes visually recede. To compensate, the ‘O’ extends fractionally beyond the cap-height and baseline. The same thing happens with lowercase ‘o’, which typically overshoots the x-height. This tiny change is an incredibly important part of optical correction.
It’s an odd way to think about letters if you’re not a type designer, but the inside of the ‘O’ matters just as much as the outside. The counter needs to remain open enough to preserve the letter’s identity, but its shape also influences the texture of a whole word/sentence/paragraph. If the counter is too narrow the typeface can feel darker, whilst a super wide counter makes a font feel flimsy. This is also why fonts are optically optimised for different sizes, even when the underlying letter design is recognisably the same. The counters of the ‘o’ are in balance with the counters of other round letters too – like ‘b’, ‘p’, ‘q’, ‘d’, ‘ce’ etc – and between the uppercase and lowercase. So it really depends on the typeface that you are drawing. If it is for text then it can’t be too narrow or too wide. But all the rest is about how the rest of the font is designed. In Latin type, the ‘o’ is one of the first letters that is tackled, together with an ‘H’ and ‘n’. That’s because it can inform so much of the design of the rest of the font – styles, axis, proportion, weight and so on. The geometry of the ‘o’, therefore, is an exercise in understanding types in general, as well as how to skew a circle (or a couple circles). Its curves, counter, stroke weight and overshoots are all carefully calibrated, and – combined – they compensate for the quirks of our odd eyes ogle ‘o’s. It is a nice reminder that type design isn’t really about making mathematically correct shapes at all. It’s much more fun than that.
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