= Typesetting line width
{title2=$W_j$}
The \b[typesetting line width] is the horizontal measure that a layout assigns to a particular line. It supplies a target for <text justification> and a fitting constraint for <line breaking>. Write it as $W_j$ when it can depend on line number $j$. The <Knuth-Plass line-breaking algorithm> permits a sequence of target widths, rather than requiring the entire <text paragraph> to be rectangular.<Breaking paragraphs into lines by Donald E. Knuth and Michael F. Plass>[pp.1155–1156: line-number dependence and supplied positive line lengths]{ref}
For example, suppose a text column is 40 units wide and <first-line indentation> reserves 10 units on its first line. Then
$$
\boxed{W_1=30,\qquad W_j=40\quad(j\geq2).}
$$
With a fixed gap of width 2, three <typesetting boxes> of widths 12, 10 and 8 use $12+2+10+2+8=34$ units. They exceed the first line's available width, although they fit a later line in the fixed-space model. When <typesetting glue> can shrink, feasibility must also account for that permitted shrinkage.
Width and <line height> measure different directions. Changing the <font size> can alter both the widths of the <typesetting boxes> and their vertical metrics, so preserving the same column width does not preserve the same breaks.
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