Leading is a hot-metal printing term that refers to the strips of lead that were inserted between text measures in order to space them accurately. Leading is specified in points and refers nowadays to the space in between the lines of a text block.
Leading introduces space into the text block and allows the characters to ‘breathe’ so that the information is easy to read.1
Leading between the lines of type is easily requested yet a difficult thing to achieve. (…) the leading has to contribute to the overall effect.
Consistent line spacing is easy to achieve with today’s typesetting machines. Every single book printed before 1770 shows how correctly a compositor could work with type and line spacing.2
The more significant the content of a book, the longer it has to be preserved, and the more balanced, indeed, the more perfect its typography has to be. Leading, letterspacing and word spacing must be faultless.
Choice of type size and of leading contributes greatly to the beauty of a book. The lines should contain from eight to twelve words; more is a nuisance. The broader margins resulting from division by nine permit a slightly larger type size than does the division by twelve. Lines with more than twelve words require more leading. Typesetting without leading is a torture for the reader.2
Legibility and readability
These two terms are often used synonymously, however, legibility refers to the ability to distinguish one letterform from another via the physical characteristics that are inherent in the design of a particular typeface, such as x-height, character shapes, counter size, stroke contrast, and type wight. The legibility of a piece of body text is enforced by the use of standard point sizes, sensitive leading and appropriate alignment. Absolute clarity of information combined with a minimum of interfering facts creates a legible type.1
Type size, line length, and interline spacing
Critical to spatial harmony and legibility is an unsderstanding of the triadic relationship of type size, line length, and interline spacing. When properly employed, these variables can improve the legibility of even poorly design letterforms or enhance the legibility of those forms considered highly legible.3
Comfortable legibility is the absolute benchmark for all typography — yet only an accomplished reader can properly judge legibility. To be able to read a primer, or indeed a newspaper, does not make anyone a judge; as a rule, both are readable, though barely. They are decipherable. Decipherability and ideal legibility are opposites. Good legibility is a matter of combining a suitable script and an appropriate typesetting method. For perfect typography, an exhaustive knowledge of the historical development of the letters used in printing books is absolutely necessary. More valuable yet is a working knowledge of calligraphy.
Typography that cannot be read by everybody is useless. Even for someone who constantly ponders matters of readability and legibility, it is difficult to determine whether something can be read with ease, but the average reader will rebel at once when the type is too small or otherwise irritates the eye; both are signs of a certain illegibility already.2
1 Ambrose, Harris. 2005. Typography. Switzerland: AVA Publishing SA
2 Tschichold, Jan. 1991. The Form of the Book: Essays on the Morality of Good Design. Vancouver: Hartley & Marks. Original edition, 1975.
3 Carter, Robert; Day, Ben; Meggs, Philip. 2002. Typographic Design: Form and Communication: John Wiley & Sons, Inc., Hoboken, New Jersey.