Typographic terms

When a designer says "the type looks too big" or "this line is cramped", there is a metric behind it inside the font file. This page walks the common terms from the baseline upwards and names the field each one comes from, so the numbers in the inspector mean something.

Baseline: where everything starts#

The baseline is the line Latin letters sit on: the bottom of a, x, m touches it, while the tails of g, p, y hang below.

Inside a font the baseline is y = 0. Every other metric is "how far above" or "how far below" it.

x-height and cap height: how big it looks#

• x-height: the height of a lowercase x. It decides how large a run of text looks — at the same 16px, a font with a large x-height reads bigger;
• cap height: the height of uppercase letters, usually a little taller.

In the font: sxHeight and sCapHeight in OS/2. ⚠️ Both are declarations and may be inaccurate or zero. For a reliable sense of visual size, look at real glyphs.

Which is why "both set to 16px, yet one looks much bigger" is normal: their x-heights differ. Font size fixes the em square, not the apparent size.

Ascender and descender: how far a line reaches#

• ascender: roughly how high b, d, h reach;
• descender: how low g, j, p go.

Two versions of this word exist, and they get confused:

Layout uses the second: reserving height for a headline or placing the next line uses the declared extents, not where one letter happens to end.

Em square and ink box#

The em square is the design grid, its side equal to unitsPerEm (often 1000 or 2048). Every coordinate counts cells in that grid, not pixels.

The ink box is the area a glyph actually occupies, usually smaller. CJK glyphs are drawn close to a square, so Chinese spacing problems cannot be solved with Latin tricks.

Letter parts#

None of these affect metrics, but they are how you judge pairing: at the same weight, different counters and terminals give a completely different character.

Where to see this here#

• Inspector → metrics draws the baseline, x-height, cap height and the hhea / typo / win extents together, with values and their percentage of one em;
• Type preview shows how x-height affects apparent size as you change the font size.

Three common misreadings#

1. "Same font size means the same size" — font size fixes the em square; apparent size comes from x-height;
2. "The em square is the glyph boundary" — it is not; glyphs often stick out (script faces especially);
3. "ascender is the tallest stroke" — it is a layout declaration, possibly higher than any glyph, possibly lower than some.

Further reading#

• OpenType spec: sxHeight / sCapHeight in OS/2
• This site: head, hhea, post, Reading the metrics