Text justification

Text justification adjusts the contents of a line to occupy its available typesetting line width, often aligning both ends of the text. Changing word spacing is one method; permissible adjustments depend on the writing system. Typographic alignment more generally includes leaving the opposite edge ragged or centring a line.^[ref]

In an illustrative line with natural line width $N=30$ and target width $W=32$, two equally flexible spaces can each gain 1. If their stretch coefficients are instead 1 and 3, proportional adjustment allocates the required extra 2 asBoth choices reach the same width; their internal spacing differs. Typesetting glue expresses that flexibility, and the line adjustment ratio computes the common proportional change.

Line breaking and text justification interact: a different break changes the number of spaces available to absorb the width difference. A good paragraph layout therefore considers spacing quality when choosing breaks, rather than merely stretching an already chosen set of lines. The Knuth-Plass line-breaking algorithm makes that interaction explicit.