Difference between point systems
A point is measure of a font size. This unit comes from old ages of typography and nowadays is slightly distorted because of "simplified" computer typesetting technique. It is usually considered to be 1/72 of an inch which is not always true.
Typography is an old art and has developed over the years a bewildering variety of mutually incompatible units. The old Roman foot/inch system did not offer originally a unit fine enough for typography, so a whole set of special purpose ad hoc units were created in various regions of the world. Some of them became obsolete while others are still used widely. The problem is that different countries use different (but similar) units adding to the great confusion...
Here is a list of different font measurement units used in different fields of typography to measure lengths and font sizes:
Two most widely used point units are ATA points (also known as Anglo-Saxon point) and Didot points. Anglo-Saxon point which is about 1/72.272" has been used on the island of the United Kingdom and on the American continent. The second point variant is the Didot point which is used in Europe. This point unit is named after the French printer François Ambroise Didot (1730 - 1804) who defined the "point-based" typographical measurement system now bearing his name.
Both systems define another unit of measurements of length equal to 12 respective points. It's pica in Angle-Saxon system and Cicero in Didot system:
It is worth mentioning that the name of the unit of Cicero comes from the ancient Roman lawyer and member of the senate, Marcus Tullius Cicero [106 BC - 43 BC) who is remembered as a master of speech and who became most famous for his disclosure of the Catilina conspiracy against the emperor of Rome.
As you can see, a point used in America is smaller than that used in Europe. This difference could be as much as 1 point for 18 point font. This poses some problems as many software products don't take this into account and are only oriented on the American market. Using these products in Europe causes the fonts in publications to become smaller than it is used to be. Some programs though support multiple measurement systems. For example, QuarkXPress allows to specify point/inch and Cicero/cm ratio. Other software like CorelDRAW allows to enter values specifying explicitly the measurement system - "pt" for ATA point and "dd" for Didot points.
Some countries are making attempts to abandon the archaic point system and use the well-established, consistent and globally accepted metric system. Metric typographic units are already used in Japan and to some degree in Germany and other European countries. However, the market dominance of US-originated typographic software without proper support for metric units at all levels currently hinders the further deployment of metric typographic practice.
Metric typography as described in the new German draft standard DIN 16507-2 works roughly as follows: absolutely everything is measured and specified in millimeters. Dimensions are multiples of 0.25 mm, or where a finer resolution is required multiples of 0.1 or 0.05 mm. No more points, picas, Ciceros, inches, etc. and all their awful conversion factors. There is nothing wrong with continued use of font specific units such as the em, as these are relative length measurements.
DIN 16507-2 defines (among many others) the following two font measures:
If we write say "Helvetica 5.0", then this means we have a font that was designed for a 5 mm line spacing. It will typically have an H that is 3.6 mm or 10.2 points tall (72% of 5 mm). Calculations become trivial: in a 60 mm high column, we can write exactly 60 mm / 5 mm = 12 lines. The baselines of text become neatly aligned with a millimeter grid, and if millimeters are used to describe both font size and font height, their relationship becomes easier to handle than if different units such as mm and points were used. Layout designers do not have to juggle any more with conversion factors such as 72.27 and 25.4. If you write "Helvetica 5.00/5.25" then this means that you use exactly the same font as above, but with 0.25 mm more baseline skip (leading) than it was designed for.
Hopefully some day we will have only one global measurement system equally accepted and used by all countries and there will be no more problems.
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