What lives inside a font file
A font file looks like magic from outside: a few hundred kilobytes that somehow teach every program on your computer a new way to write. Inside, it's closer to a well-organised filing cabinet — a handful of tables, each answering one question. Knowing the four big ones makes the press on this site stop being a black box, and explains most of the quirks you'll ever meet in any font, purchased ones included.
The em square: every letter's plot of land
Fonts don't store letters in pixels or millimetres. Each glyph is drawn inside an abstract square — the em — traditionally divided into 1,000 units. "12-point text" means: scale the em square to 12 points, and everything drawn inside scales along. The baseline sits partway up this square; ascenders reach toward its top, descenders hang below the line. When the press maps your pad's guide lines into font units, it's deciding where in this square your writing lives — which is why letters drawn wildly above the cap line come out looking oversized in text: they genuinely claimed more of the em than letters usually do.
Outlines: shapes, not strokes
The part that surprises everyone: fonts contain no pen strokes. A glyph is stored as closed outlines — the boundary of the inked region — which renderers fill like a stencil. The middle line of your pencilled 'l' doesn't exist in the file; what exists is a long thin filled loop around where the ink was. This is why the press does real geometry on your strokes: it walks along each one laying out the left edge, rounds a cap at the end, walks back along the right edge, and closes the loop. Your movement becomes a boundary. It's also why pen pressure can't survive — the outline records where ink was, not how hard you leaned.
Advance widths: the invisible sidecar
Each glyph carries a number called its advance width: how far the typing position moves after it's placed. It includes breathing room on both sides — the side bearings — and it's what makes text flow without letters colliding. When your proof sheet shows an 'i' hogging space or an 'm' cramped against its neighbours, you're seeing advance widths inherited from how wide you drew; the fix is redrawing, and now you know what you're fixing. Grown-up fonts add a further table of kerning pairs ("pull T and o closer together"), which is exactly the kind of obsessive polish a one-afternoon handwriting font cheerfully skips.
The cmap: the phone book
Finally, something has to connect the letter you type to the shape in the file. That's the cmap — a lookup from character codes to glyphs. It's where the press quietly implements the capitals fallback: if you never drew 'B', the cmap simply lists your small 'b' as the answer for both codes. Same trick professional fonts use for making 'A' and 'Á' share a base shape. When a font "doesn't have" a character and shows a hollow box, that's a cmap with no entry — the filing cabinet's polite way of saying nobody filed that one.
That's honestly most of it: a coordinate system, filled boundaries, spacing numbers and a phone book, zipped into one binary. The press builds all four from your strokes, in your tab, in about a tenth of a second — which is either mundane or miraculous, depending on how recently you've thought about what a font actually is.