= Cap height
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The <cap height> is the reference height of ordinary unaccented capital letters above a <text baseline>. It describes uppercase letter design, while the <x-height> concerns the main lowercase body. A <font ascent> can reserve more space for accents or other tall forms than the <cap height> alone requires.
<OpenType> records a nominal <cap height> in the `sCapHeight` field of its `OS/2` metrics table, in <font units>. The metric is an approximation for the uppercase design, so taking the largest ink <bounding box> among every uppercase <glyph> need not give the intended value.<OS 2 and Windows Metrics Table by Microsoft>[sCapHeight: approximate height of unaccented uppercase letters]{ref}
For a hypothetical <computer font> with cap metric 700 and 1000 <font units> per <em>, the reference height at 18 <pixels per em> is 12.6 <pixels> before <grid fitting>. An ascent metric of 800 <font units> would instead reserve 14.4 <pixels> above the <text baseline>. These describe different geometric features of the same design.
Aligning a capital heading against an icon can therefore require choosing which reference matters: the <text baseline>, the nominal <cap height>, or a measured ink <bounding box>. Keeping those quantities separate makes the alignment rule reproducible.
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