This article deals with the typographic meaning of "leading". For the management meaning, see leadership.
In typography, leading (IPA [ˈlɛdɪŋ], rhymes with heading) refers to the amount of added vertical spacing between lines of type. In consumer-oriented word processing software, this concept is usually referred to as "line spacing." Leading may sometimes be confused with tracking, which refers to the horizontal spacing between letters or characters.
The word comes from lead strips that were put between set lines. When type was set by hand in printing presses, slugs or strips of lead (reglets) of appropriate thicknesses were inserted between lines of type to add vertical space, to fill available space on the page.
Text set "solid" (no leading) appears cramped, with ascenders almost touching descenders from the previous line. The lack of white space between lines makes it difficult for the eye to track from one line to the next, and hampers readability.
The following block of text has no leading:
This block of text set with 50% leading is easier to read:
This block of text at 100% leading is also easy to read, but makes less efficient use of vertical page space:
In CSS, leading is implemented by the line-height property.