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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.^[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.

The font ascent is an upward metric measured from a text baseline and used when allocating vertical space for a font. It describes the font's layout metrics as a whole, rather than the visible ascender of one particular glyph. A short letter, a tall letter and an accented form can therefore share the same font ascent while having different ink bounding boxes.^[ref]

Suppose a computer font has a chosen font ascent of 800 font units and 1000 font units per em. At 20 pixels per em, the scaled upward metric is 16 pixels. This calculation allocates a reference extent; it does not assert that the letters being drawn actually reach 16 pixels above the text baseline.

The field used must be stated. OpenType provides ascender in hhea and sTypoAscender in OS/2, while some older layout paths use other metrics. The sTypoAscender, sTypoDescender and sTypoLineGap fields are intended to work together for typographic line spacing. Their ascent-minus-descent need not equal unitsPerEm.^[ref]

Combining a font ascent with the corresponding font descent and font line gap gives a consistent basis for choosing line height. Mixing fields chosen under different conventions can instead change the spacing even when the nominal font size is unchanged.

A font line gap is a font-wide metric giving an additional spacing allowance to combine with font ascent and font descent. For an elementary default line-spacing calculation using signed descent $d_f$,where $a$ is the ascent and $g$ the font line gap. OpenType's sTypoLineGap is intended to be used with the matching sTypoAscender and sTypoDescender fields.^[ref]

Suppose the chosen metrics are $a=760$, $d_f=-240$ and $g=100$ font units. The default spacing is 1100 font units. With 1000 font units per em at 18 pixels per em, that is 19.8 pixels between text baselines.

The stored font line gap provides an input to a spacing decision. An application can instead request a different line height, and CSS leading is then calculated from that requested height and the selected metrics. Nor does the font line gap measure the visible blank strip between every pair of lines: the ink bounding boxes vary with the particular glyphs. The line height diagram shows this distinction with schematic ink inside larger font-wide metric regions.