When I first watched a sign painter hand lettering, I was spellbound by his ability to shape beautifully consistent letter forms with a quill, his name for a lettering brush. He didn’t draw the letters first, other than some quick marks for placement. I also noticed that he made the round letters, like the ‘S’ and the ‘O,’ slightly taller than the straight letters. The round letters protruded above and below the lines. Thinking these were mistakes at first, I soon saw he was making the letters bigger intentionally. When I asked him why, he said it was “so they would all look the same height.”
So began my introduction to the many optical illusions involved in lettering. Since then I have learned that there is even a typographical name for the degree to which a round letter extends beyond the lines—it’s called “overshoot.” And I have learned there are many other optical illusions at work in typography, so that frequently things need to be drawn differently to make them look the same.
When round letters are placed next to straight letters of the exact same height, the round letters will always look shorter. Why is this? It seems to be because less of the round letter reaches it tallest point. The rest of the letter falls rapidly away from this high point, so that most of the letter actually is shorter, leading our brain to perceive the round letter as shorter overall. The exact mathematical height of a letter is not as important as how tall it appears. When I was an apprentice, I was told, “It doesn’t matter what the yardstick says. If it looks right, it is right.” In the words of noted typographer Tobias Frere-Jones, “we read with our eyes, not with rulers, so the eye should win every time.” Consequently, round letters must be made slightly taller to compensate for the optical illusion.
How much is the overshoot of round letters? While 1-3 percent of the height is common, the overshoot varies with the letterstyle (and with type designer). Letters with wider, more gradual curves will show less overshoot than letters with narrower, tighter curves. A letterstyle like Eurostile, for example, with its squarish letters, has little overshoot. In fact, from a distance it’s hard to tell the caps apart from each other—they all resemble soft squares. On a sign, Eurostile, especially the upper case, is not the best choice, and must be used carefully to avoid illegibility.
For a well-articulated explanation of overshoot, with illustrations, see Frere-Jones’ post: