Typographic unit

Typographic units are the units of measurement used in typography or typesetting. Traditional typometry units are different from familiar metric units because they were established in the early days of printing. Though most printing is digital now, the old terms and units have persisted.
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, widows and orphans, disrupted tables, and misplaced captions). Before the popularization of desktop publishing, type measurements were done with a tool called a typometer.
Development.
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.
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. 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:"
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.).
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 "12 dd" means twelve didot points.
In Britain and the United States, 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 nationwide standard was created until the American Point System was decided in 1886.
The American Point System was proposed by Nelson C. Hawks of Marder Luse & Company in Chicago in the 1870s, and his point system used the same method of size division as Fournier's; viz. dividing 1 inch by 6 to get 1 pica, and dividing it again by 12 to get 1 point. However, the American Point System standardized finally in 1886 is different from Hawks' original idea in that 1 pica is not precisely equal to 1⁄6 inch (neither the Imperial inch nor the US inch), as the United States Type Founders' Association defined the standard pica to be the Johnson Pica, which had been adopted and used by Mackellar, Smiths and Jordan type foundry (MS&J), Philadelphia. As MS&J was very influential in those days, many other type foundries were using the Johnson Pica. Also, MS&J defined that 83 Picas are equal to 35 centimeters. The choice of the metric unit for the prototype was because at the time the Imperial and US inches differed in size slightly, and neither country could legally specify a unit of the other.
The Johnson Pica was named after Lawrence Johnson who had succeeded Binny & Ronaldson in 1833. Binny & Ronaldson was one of the oldest type foundries in the United States, established in Philadelphia in 1796. Binny & Ronaldson had bought the type founding equipment of Benjamin Franklin's (1706–1790) type foundry established in 1786 and run by his grandson Benjamin Franklin Bache (1769–1798). The equipment is thought to be that which Benjamin Franklin purchased from Pierre Simon Fournier when he visited France for diplomatic purposes (1776–85).
The official standard approved by the Fifteenth Meeting of the Type Founders Association of the United States in 1886 was this Johnson pica, equal to exactly 0.166 inch. Therefore, the two other – very close – definitions, 1200 / 7227 inch and 350 / 83 mm, are both unofficial.
Monotype wedges were made with an accuracy of 10.000 dpi. Wedges used in England and America were based on a pica = .1660 inch. But on the European continent all available wedges were based on the "old-pica" 1 pica - .1667 inch. These wedges were marked with an extra E behind the numbers of the wedge and the set. These differences can also be found in the tables of the manuals.
"In the American point system:"
The American point system has been used in the US, Britain, Japan, and many other countries.
Today, digital printing and display devices and page layout software use a unit that is different from these traditional typographic units. On many digital printing systems (desktop publishing systems in particular), the following equations are applicable (with exceptions, most notably the popular TeX typesetting system and its derivatives).
Fournier's original method of division is now restored in today's digital typography.
Comparing a piece of type in didots for Continental European countries – 12 dd, for example – to a piece of type for an English-speaking country – 12 pt – shows that the main body of a character is actually about the same size. The difference is that the languages of the former often need extra space atop the capital letters for accent marks (e.g. Ñ, Â, Ö, É), but English rarely needs this.
Metric units.
The traditional typographic units are based either on non-metric units, or on odd multiples (such as 35⁄83) of a metric unit. There are no specifically metric units for this particular purpose, although there is a DIN standard sometimes used in German publishing, which measures type sizes in multiples of 0.25 mm, and proponents of the metrication of typography generally recommend the use of the millimetre for typographical measurements, rather than the development of new specifically typographical metric units. The Japanese already do this for their own characters (using the "kyu", which is "q" in romanized Japanese and is also 0.25 mm), and have metric-sized type for European languages as well. One advantage of the q is that it reintroduces the proportional integer division of 3 mm (12 q) by 6 & 4.
During the age of the French Revolution or Napoleonic Empire, the French established a typographic unit of 0.4 mm, but except for the government's print shops, this did not catch on.
In 1973, the "didot" was restandardized in the EU as 0.375 (= 3⁄8) mm. Care must be taken because the name of the unit is often left unmodified. The Germans, however, use the terms Fournier-Punkt and Didot-Punkt for the earlier ones, and Typografischer Punkt for this metric one.