The Truchet point, the first modern typographic point, was 1/144 of a French inch or 1/1728 of the royal foot. It was invented by the French clergyman Sébastien Truchet. During the metrication of France amid its revolution, a 1799 law declared the meter to be exactly 443.296 French lines long. At 9000 lines or 16 inches per foot, this established a length to the royal foot of 9000/27706 or ca. 325 mm, which made the Truchet point equal to 15625/83118mm or about 187.986 µm, although it has also been cited as exactly 188 µm.
The Fournier point established by Pierre Simon Fournier in 1737 was about 11/864 French inches or (by 1799) 345 µm. This is very close to the present international point, but Fournier's point did not achieve lasting popularity despite being revived by the Monotype Corporation in 1927. It became standard in Belgium.
The system of Fournier was based on the Paris foot, 1 foot = 12 paris inches = 12 * 12 = lignes = 12 * 12 * 12 points. Two of these points became the Fournier-point. Fournier did cast his type on multiples of 3 points Fournier.
The Didot point established by François-Ambroise Didot in 1770. was twice Truchet's and thus 1/864 of the royal foot, i.e. by 1799 15625/41559mm or ca. 375.971 µm.
12 point Founier equaled 11 point Didot, because of this the Fournier system was later called "median-system". This name "median" was used for type at 11 point.
Approximations were subsequently employed, largely owing to the Didot point's unwieldy conversion to metric units. (The divisor of its conversion ratio has the prime factorization of 3×7×1979.) Values included Hermann Berthold's 376 µm point and Jan Tschichold's 375.94 µm (266 points to 100 mm). Due to the definition in TeX of 1157 dd = 1238 pt, the slightly larger 376.065 µm became a common value.
TeX also supports a new Didot point (nd) at 3/8mm or 375 µm, and cites a 1978 redefinition for it. The French National Print Office adopted a point of 2/5mm or 400 µm exactly and continues to use this measurement today. Japanese and German standardization bodies instead opted for a metric typographic base measure of exactly 1/4mm or 250 µm. It is called Q in Japanese after the initial letter of quarter millimetre. Due to demand by Japanese typesetters, CSS adopted Q in 2015.
The Didot point has been mostly replaced by the DTP point in Europe and throughout the world.