Point size

A point is a seventy-second of an inch, and the point size of type is the height of the body the letters are drawn on, not the height of anything you can see. The specimen above draws that body as a box: the box is exactly as tall as the number says, the capitals stop short of the top, the lowercase stops well short of that, and the descenders hang below the line. Change the number and the box changes by exactly that much. Everything inside it changes by whatever proportion the type designer chose.

That is a direct inheritance from metal. A foundry cast each size on a physical rectangle of lead, and the point size was the height of that rectangle, which had to hold the tallest ascender and the deepest descender of the design with a little clearance. How much of the block the letters actually filled was the punchcutter’s call, so two faces cast on a 12 point body could look very different sizes on the page. Nothing about that changed when type went digital: the block became the em, the em is exactly the font size, and the x-height and cap height inside it are still the designer’s decision. This is the whole answer to why the same number in two faces does not look like the same size, and why substituting a fallback font at a matching point size can still change a layout.

On the web you rarely type pt at all. CSS defines it as an absolute unit, exactly 1.3333px, which makes it a clumsy way of saying something rem says better, and print stylesheets are about the only place it still earns its keep. The word survives everywhere else because the vocabulary did: designers hand over specs in points, native platform documentation talks in points, and Apple’s “points” are a display unit rather than a typographic one, which is a collision worth naming out loud before a handoff rather than after it. Leading is quoted in the same currency, historically as a pair such as 10 on 12, and a drop cap is one of the few things sized in lines rather than in points.

The practical consequence is to stop trusting the number on its own. Two faces that need to look equally large will usually need different point sizes, matched by eye on their x-heights rather than by their nominal size, which is exactly what a designer is doing when they bump a substituted face up a point. And because the reader can override your sizes and should be able to, set body text in a relative unit and let resizing text work rather than nailing a point size to the page.