LEADING (PRONOUNCED "LEDDING") is the space between the lines of type. In the days of hot metal typesetting this space was created by inserting thin strips of lead between the lines. Lines of type without these strips of lead wereand still arereferred to as "set solid." Leading plays a big part in the readability of your text. All body text is made more readable by a positive amount of leading (a leading value greater than the point size of the type). Headlines and display type, on the other hand, may benefit from negative leading (a leading value less than the point size of the type).
InDesign regards leading as a character level format (as opposed to a paragraph format in QuarkXPress). Leading can be applied "locally" to a selected range of text using the Control palette or the Character palette, or "globally" as part of a style sheet definition.
Figure 4.1. Basic Character Formats.
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Bad leading makes your text harder to read because the eye has trouble locating the next line of type. When leading is too tight, the type may appear intimidatingly dense, and the descenders of one line will collide with the ascenders of the next. At the other extreme, when the leading is too loose the type lack cohesion. This is especially so if the leading is greater than the space between the paragraphs.
Leading is measured in points from one baseline to the next. The leading value includes the point size of the typeface and the actual space between the lines. Thus, 12 points of leading using 10-point type really means two points of space between lines.