Typographic units are the units of measurement used in typography or typesetting. The traditional units are different from common metric units, as they were established earlier. Even though these units are all very small, across a line of print they add up quickly. Confusions such as resetting text originally in type of one unit in type of another will result in words moving from one line to the next, resulting in all sorts of typesetting errors (viz. rivers of white, widows and orphans, disrupted tables, and misplaced captions).
In Europe, the Didot point system was created by François-Ambroise Didot (1730–1804) in c. 1783. Didot’s system was based on Pierre Simon Fournier's (1712–1768), but Didot modified Fournier’s by adjusting the base unit precisely to a French Royal inch (pouce), as Fournier’s unit was based on a less common foot.
(Fournier’s printed scale of his point system, from Manuel Typographique, Barbou, Paris 1764, enlarged)
However, the basic idea of the point system – to generate different type sizes by multiplying a single minimum unit calculated by dividing a base measurement unit such as one French Royal inch – was not Didot’s invention, but Fournier’s.[note 1] In Fournier’s system, an approximate French Royal inch (pouce) is divided by 12 to calculate 1 ligne, which is then divided by 6 to get 1 point. Didot just made the base unit (one French Royal inch) identical to the standard value defined by the government.
In Didot’s point system:
- 1 point = 1⁄6 ligne = 1⁄72 French Royal inch = 15 625⁄41 559 mm ≤ 0.375 971 510 4 mm, however in practice mostly: 0.376 000 mm, i.e. + 0.0076 %.
Both in Didot’s and Fournier’s systems, some point sizes have traditional names such as Cicero (before introduction of point systems, type sizes were called by names such as Cicero, Pica, Ruby, Long Primer, etc.).
- 1 cicero = 12 Didot points = 1⁄6 French Royal inch = 62 500⁄13 853 mm ≤ 4.511 658 124 6 mm, also in practice mostly: 4.512 000 mm, item: + 0.0076 %.
The Didot point system has been widely used in European countries. An abbreviation for it that these countries use is "dd", employing an old method for indicating plurals. Hence "12dd" means twelve didot points.
In Britain and the U.S.A., many proposals for type size standardization had been made by the end of 19th century (such as Bruce Typefoundry’s mathematical system that was based on a precise geometric progression). However, no nation-wide standard was created until the American Point System was decided in 1886.
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