Glyph design: the lowercase o

Once you've made the ‘a’, you should go on to make the next most important letters: ‘i’ and ‘o’. Almost every letter of the lowercase alphabet can be derived from ‘a’, ‘i’, and ‘o’.

o
i
a

b
l
n

d p q
e
f
v
h u m
r

c
j
t
y
w
x

k
z

g
s

This post will be on how to construct the letter ‘o’.

As far as the lowercase alphabet goes, ‘o’ is one of the simpler letters. The important features of the letter are shown below:

Components of a lowercase letter 'o'

Bowl

     The curved part of the ‘o’. So basically, the whole letter.

Overshoot and undershoot

     These aren’t technically parts of the letter, rather how much the ‘o’ extends above the mean line (generally the height of a lowercase x and most of the lowercase letters) or the baseline. This is done to make the ‘o’ look the same size as other flat-topped letters, because of an optical illusion that makes circles look smaller than rectangles of the same height.

     There are also a few other features that aren't strictly letter parts, but are still integral to the letter's structure:

Axis

     An imaginary line that runs parallel to the thickest part of the ‘o’ and intersects it at the thinnest parts. In calligraphic terms, it's perpendicular to the nib angle.

Counter

     The empty space in a letter—the hole in the ‘o’. The terms comes from counterpunch, a block of metal that was used to make the holes in the letters back in the days of metal type casting. Be sure not to confuse a counter with a contour—which is simply a closed path that outlines a letter. The letter ‘o’ has two contours, but only the inner one is a counter.

The o is not a circle 

People usually assume the ‘o’ is a circle. This is wrong, though not egregiously wrong when it comes to typographic misconceptions. When it comes to very-old style ‘o’s, the outer contour can actually really be thought of as a circle. But most ‘o’s are slightly thinner than a perfect circle. The counter (inner contour) of the letter is almost never a perfect circle—not even in most geometric sans serifs.

Garamond, Minion, and Proforma, from left to right. Perfect circles are in pink.

The science and math-minded among you might be wondering if the ‘o’ can then be constructed with an ellipse.

Well, sorry to burst your ellipse, but the answer is no. 

The same three ‘o’s, with their ellipses of best fit in pink. Outlines are shown to make the differences easier to see.

The ‘o’ is slightly more squarish than an ellipse. This probably came from the fact that it’s impossible to draw a perfect calligraphic circle or ellipse, but it likely also has something with the fact that ellipses and circles tend to appear slightly rhombus-like to the human eye, especially when next to squares and rectangles.

The top three ‘o’s have elliptical outer contours, the lower three are what they actually look like in their respective typefaces. Even though the top three are mathematically rounder, the three on the bottom look rounder to the human eye.

Also note that the upper right and lower left corners of the ‘o’ tend to stick out a bit more than its other two corners.

There’s no way to construct a shape like that from geometric primitives, but you can convert an ellipse to a path and tweak its béziers to form it manually.

In yellow is a perfect ellipse, in pink is a modified bézier path.

The outer contour of the ‘o’ can be crudely thought of as an upright oval (though its axis is actually usually tilted some number of degrees clockwise—the reason the top-right and bottom-left corners deviate more from an ellipse than the other two corners). But the inner contour is almost always a tilted oval, except in the most didone serif ‘o’s. The tilt of that white oval (sometimes up to ten degrees) is what determines the letter’s axis.

The tilts of the counter ovals in Garamond, Minion and Proforma. Although no one would ever call Proforma a didone serif, it does have a near-vertical stress (pen angle), giving it an almost vertical axis.

Of course, the ovals are not ellipses, not even in Garamond.

So while a tilted ellipse is a good starting point, as always some manual bézier tweaking is needed to get an ‘o’ that really looks good.

Now that we have two letters drawn, it’s a good idea to make sure they were drawn the same weight (thickness). In fact, it's not a bad idea to check every time you draw a new glyph. When we compare the stem of the ‘a’ with the thickest part of the ‘o’, we find that the two widths are roughly the same length. In fact, the ‘o’ is slightly heavier.
 
You would think that you should make sure that the thickest and thinnest parts of the ‘o’ should be the same thickness as the thickest and thinnest parts of the ‘a’. But here’s something you might not have known: The ‘o’ actually has to be quite a bit heavier than the ‘a’ to look right. That’s because of an optical illusion that makes curves look thinner than straight stems. That makes sense—if the thickest part of the ‘o’ is a certain width, the ‘o’ is only that thick at that point—the ‘o’is thinner everywhere else. The stem of the ‘a’ is, on the other hand, at that maximum thickness for its entire length. So the maximum width of the ‘o’ has to be made a bit thicker to compensate (according to some designers, up to 20 percent). The thin part of the ‘o’ should also be made slightly thinner than the thinnest part of the ‘a’, since the ‘a’s hairline is a bit straighter than that of the ‘o’.

And with that correction, our ‘o’ is complete!