Units - Metric Typographic Units

Typography is an old art. Long before the introduction of the international standard system of units (“metric system”), printing equipment manufacturers all over the world have established a bewildering variety units to measure length, many of which continue to be used today:

· 1 point (Truchet) = 0.188 mm (obsolete today)
· 1 point (Didot) = 0.376 mm = 1/72 of a French royal inch (27.07 mm)
· 1 point (ATA) = 0.3514598 mm = 0.013837 inch
· 1 point (TeX) = 0.3514598035 mm = 1/72.27 inch
· 1 point (Postscript) = 0.3527777778 mm = 1/72 inch
· 1 point (l’Imprimerie nationale, IN) = 0.4 mm
· 1 pica (ATA) = 4.2175176 mm = 12 points (ATA)
· 1 pica (TeX) = 4.217517642 mm = 12 points (TeX)
· 1 pica (Postscript) = 4.233333333 mm = 12 points (Postscript)
· 1 cicero = 4.531 mm = 12 points (Didot)

The printing and publishing software market is at present dominated by manufacturers (Apple, Adobe, Microsoft, Quark, etc.) located in the United States, the last country on the planet that has yet to make significant progress towards the introduction of modern standard units. As a result, the use of standard units is far from well established in digital typography, to the significant annoyance of users all over the world.

The main problems are:

• The dominant unit of length, the Postscript point, has with 25.4/72 = 0.352777... mm a very inconvenient relationship to the most widely used length units (meter and millimeter).
  
• There exists no well-established practice for denoting a font size. [One example of a somewhat established convention is to specify the length of an “em” in PostScript points. Historically, the “em” was the width of the widest metal type in a font, which was for Roman fonts typically the capital letter “M”. Today, the control points of digital font outlines are stored in terms of coordinates inside a unit square. This square is a vague equivalent of the historic maximum metal type size and its side length has become the modern incarnation of the “em”. As a result, no easily measurable dimension in a text matches the point length that designates a font size]
  
• Resolutions of output devices are still frequently specified in dpi (dots per inch), which is the reciprocal value of the pixel size multiplied with 25.4 mm.

With the metric system, we have now a well established, consistent, and globally accepted set of length units, ranging from subatomic to cosmological dimensions. The use of archaic ad-hoc special purpose units has become obsolete and should be strongly discouraged.

It is time that the typographic community finally abandons its current unit mess in two ways:

• Adopt the metric system
  
• Denote font sizes based on a measurable characteristic length of the printed glyphs
  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.

The German draft standard DIN 16507-2 suggests that all length measurements in digital typography should be specified in millimeters. It suggests further that dimensions should be 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.
Japanese typesetters use the unit Q (quarter) for font sizes, where 1 Q = 0.25 mm, i.e. the same modulus recommended by DIN 16507-2.

This measure coincides nicely with the most common pixel size on computer monitors. For example a typical CRT screen has a display area of 320×240 mm, divided into 1280×1024 pixels, which makes each pixel 0.25 mm large.