First-line indentation

First-line indentation shifts the beginning of the first formatted line of a text paragraph relative to its normal starting edge. In a left-to-right layout, a positive indent reserves space on the left; the corresponding starting edge is on the right in a right-to-left layout. In CSS, text-indent controls this placement.^[ref]

In a simple left-to-right column of width $W=40$ with indentation $d=10$, the first line begins at $x=10$ and has available typesetting line width $W-d=30$. Later lines can begin at $x=0$ and use all 40 units. A negative indent $d=-5$ instead places the start at $x=-5$, extending into the margin if no other layout constraint intervenes.

This changes the first line's geometry without adding printable glyphs. Replacing the indent by several stored spaces would couple the result to word spacing, white-space processing and the chosen computer font. A line breaking model can represent the change explicitly through its sequence of typesetting line widths.