 Text paragraph

A text paragraph is a group of text treated as one unit for paragraph layout. Its line breaking may change when the available typesetting line width or computer font changes, while its underlying paragraph boundary remains the same. A line in the source file therefore need not be a line of displayed output.^[ref]

For example, a sequence of four typesetting boxes may occupy two lines in a narrow container and one line in a wider container. Reflow changes the grouping of boxes without requiring a new paragraph. In TeX, \par ends the current paragraph; in HTML, a p element marks a paragraph. Those mechanisms let the layout system work with a semantic group instead of relying on the writer to insert each visible line break.

The Knuth-Plass line-breaking algorithm uses this grouping to compare costs across the whole paragraph. Its typesetting demerits can depend on both individual lines and changes between neighbouring lines, which cannot be evaluated by inspecting each source line in isolation.