From Tobias Frere-Jones' new series on typeface mechanics:
Square shapes like H have a simple and stable relationship to the baseline and cap height. Their upper and lower edges coincide with these boundaries and stay put. But only a narrow sliver of an O is the full height, and the rest of the shape falls away. The parts that are too short greatly outnumber the parts that are big enough, so we conclude — wrongly, but very reliably — that the round shape is too small.
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.
It's details like this that make good products. Details that you'd never notice on their own, but ones that all add up to create a refined and effortless result. Without them, you get mediocrity. Good enough. Functionality that doesn't quite feel right.
These are the kind of details I spend a lot of my working days considering. Some people consider it overthinking. To me, it's an essential part of the process. If you think a detail doesn't matter, you're not doing justice to your product and you're letting your customers down. Charles Eames put it very directly:
The details are not the details. They make the design.
I can't wait to read the rest of this series.