= Leading
{disambiguate=typography}
= Typographic leading
{synonym}
<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>.<A Glossary of Typographic Terms by Adobe>[Entry leading: line spacing and the historical lead spacers]{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 $H$, then
$$
\boxed{L=H-(A+D).}
$$
The 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>.<Visual formatting model details in CSS 2.2 by W3C>[Section 10.8.1: A, D, leading L and half-leading]{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.
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