A measurement system used in typography. The system dates from early handset metal type, where the sizes of type cast by type founders were graduated on a uniform "point" scale. Each size is described by its number of points (called its point size; see Point Size), which referred to the height of the body on which it is cast. Calculations are simplified by assuming each point is one-seventy-second of an inch. At one time, there were three point systems in use worldwide:
American-British System. This is the point system used throughout North America and Great Britain. The standard of measurement is the 0.166-inch pica and the 0.01383-inch point. One point is equal to one-twelfth of a pica. Thus, 1,000 lines of pica—or 12-point—matter measure 166 inches, and 1,000 lines of 6-point matter measure 83 inches. Point is abbreviated pt.
Didot System. Used primarily in Europe, the Didot system uses the cicero as its basic unit, which is equal to 12 corps, or 0.178 inch. The Didot corps (or Didot point) measures 0.0148 inch.
Mediaan System. Once used principally in Belgium, the Mediaan system has a corps (or point) equal to 0.01374 inch. The Mediaan em, or cicero, measures 0.165 inch. The Mediaan system, however, has largely been replaced by the Didot system.
For general, practical measurement purposes, three decimal places (thousandths of an inch) are deemed sufficient significant digits.
In the American-British system, it is convenient to remember 6 picas equal one inch, 12 points equal one pica, and 72 points equal one inch—but it's not exactly true. Picas and points do not have an exact relationship to inches. Thus, 30 picas—or approximately 5 inches—actually equal 4.98 inches.
It should be noted, however, that much of Europe is moving to purely metric measurements.