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These fonts are all the same size. Except, they’re not.

When we size fonts with CSS, what we’re sizing is the bounding box that surrounds our type. This box (the em box) is a built-in part of font files. Type designers decide how glyphs are sized and positioned inside. But to web designers it's more of a black box. We have no way of measuring the glyphs themselves.

Tools should use font metrics to help us make design decisions. Suppose we had a typeface with a small x-height, a wide stance, or very long extenders — with font metrics, tools could provide size, line length, and line height recommendations to give us a typographic head start.

Keep an eye on Typekit Practice, and let’s talk about how font metrics might be useful in our design work. — Tim BrownGreg Veen, and Bram Stein