Leads

In typography, leading (/ˈlɛdɪŋ/ LED-ing) refers to the distance between the baselines of successive lines of type. The term originated in the days of hand-typesetting, when thin strips of lead were inserted into the forms to increase the vertical distance between lines of type. The term is still used in modern page-layout software such as QuarkXPress and Adobe InDesign.

In consumer-oriented word-processing software, this concept is usually referred to as "line spacing" or "interline spacing".

The word comes from lead strips that were put between set lines of lead type, hence the pronunciation "ledding" and not "leeding". When type was set by hand in printing presses, slugs, or strips of lead of appropriate thicknesses were inserted between the lines of type to add vertical space, improving legibility.

Leading can be used to enhance the readability of a page or block of text. The standard leading used in printing is usually +2 points for mechanically set body text, or +20% in digital typesetting. In both cases, with a 10 pt font the usual leading is 12 pt, creating a 10/12 ratio, which is considered single spacing. Some type designers recommend a leading between 120% and 145% of the font size.

Double spacing is an entrenched practice due to the era of typewriters and, in academic settings, to allow the addition of handwritten comments and proofreading. Typewriters had a limited number of options for leading, and double spacing was chosen as a default. Double spacing can increase the amount of unused white space on a page and lessen the number of lines on a page. Too much leading can cause continuity problems, as the eyes of the reader are required to travel a greater distance between lines of text.

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Wikipedia