Take a CLOSE LOOK at this sentence. The capital letters look like they're the same height, and the the lower case letters look like they're the same height. But zoom in and you'll see that's an optical illusion. Letters like the capital "O" might stand a little taller than the letter "L" or "E." They're cheating. But why?
As it turns out, almost all typefaces vary in height from one letter to the next, for a very simple reason: when characters in a typeface are all the exact same height, they don't actually look like they're the same height, so type designers have to cheat. In a new post on his blog, designer and type historian Tobias Frere-Jones explains the optical illusions of typography.
Curved characters like "C" and "O" tend to look shorter than squarish characters like "H" or "T" at the same height, because less of the character touches the tallest point on the line. Therefore, type designers tend to overshoot, trading mathematically equal height for optically equal height.
Frere-Jones explains:
If the "correct" height appears inadequate, "too much" will look right. So the O is made taller and deeper than the H, even if the most stringent mathematical reasoning would declare it incorrect. But we read with our eyes, not with rulers, so the eye should win every time. Typefaces from any period will demonstrate this compensation, often called "overshoot."
If curves need overshoot because they don't behave like squares, pointed shapes are even less like squares, and accordingly get more overshoot. But with enough care, readers won't notice the multiple sizes and positions. All of it will feel equal.
As Frere-Jones points out, it's just one of the many little problems designers face that make coming up with a new typeface a lot more art than it is science. But there's much more. Make sure to read the whole thing.