Font line gap

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.