Leading (typography)

Typographic leading concerns vertical spacing between lines of type. The term comes from the thin lead spacers used to separate lines of metal type. In current discussion, one must establish whether a quoted value means the complete spacing between text baselines or an additional allowance beyond the chosen font ascent and font descent.^[ref]

CSS leading uses the additional-allowance convention. If a single font contributes ascent $A$ and descent magnitude $D$, and the chosen line height is $H$, thenThe elementary inline-box construction allocates half of $L$ above the ascent and half below the descent. These quantities are font-wide metrics, rather than the actual ink bounding box of each glyph.^[ref]

For hypothetical metrics $A=11$, $D=3$ and $H=18$ CSS pixels, the extra leading is 4 CSS pixels, with 2 on each side. If the same metrics use $H=12$, then $L=-2$: the allocated line region is smaller than the metric extent. Negative CSS leading is allowed, so visible ink can extend outside its line allocation. A line height value must consequently be interpreted together with the font and the layout convention.