A little history of metric thread standards (cited from Screw thread standards for federal services 1957, National Bureau of Standards H28, 1957):
( Slightly of topic)
"Metric-thread systems have been used in European Continental countries since 1848, particularly in France, Germany, and Switzerland. Efforts toward international unification of these systems led in 1898 to a conference in Zurich, Switzerland, which was attended by representatives from engineering societies and other technical organizations in France, Germany, Italy, the Netherlands, and Switzerland. Organizations in other countries such as the United States and Great Britain, were also invited but did not send delegates.
The Zurich Conference of 1898 adopted a system of metric threads which was practically the same as that previously developed in France by the Société d'Encouragement pour l'Industrie Nationale in 1894. This system became known as the "International System" and is usually designated as the SI System" (from the French name, "
Système Internationale"). This system was recommended for adoption by all countries where metric threads were used and covered a range of nominal diameters from
6 to 80 mm, inclusive, with associated (coarse) pitches. The threads were intended for use as fastening threads in machine construction and hence for application to the general types of screws, bolts, and nuts.
The need for metric coarse threads in sizes smaller than 6 mm and Iarger than 80 mm, and of metric fine threads, led a number of Continental European countries to extend the original SI series. However, these additional series showed differences in respect to nominal diameters, pitches, and diameter-pitch combinations. Nationai standardizing bodies, organized in Europe during and after the first World War, made an effort to bring some order in these additional series. In 1926 the International Standards Association (ISA) was founded and one of its first technical committees dealt with metric threads. At a conference held in Copenhagen in 1931, this committee succeeded in getting agreement in principle on five recommended series of metric threads, designated by the letters A to E. It took several more years to put the final touches on this unification plan, and ISA Bulletin 26, in which the recommended ISA series are listed, was not published until September, 1940. The original series of SI coarse threads was extended to diameters as large as 600 mm (about 24 in.), the pitch being 6 mm for all sizes above 80 mm. Therefore, the term "coarse" threads was avoided and the original SI series, with its upward downward extensions, was designated as "ISA Series A." However, ISA Bulletin 26 and the national standards set up in accordance with it, explicitly refer to the ISA Series B to E, inclusive, as "fine threads." The ISA became inactive in 1942 as a result of the second world war. Following the war the International Organization for Standardization (ISO) was established, and ISO/TC1, Screw Threads, held its first meeting at Paris in 1949. This technical committee subsequently developed recommendations for basic and design thread profiles, and standard series for metric and inch screw threads."
The French National Standard metric threads below 6 mm diameter always were called "S.I.", though they were not covered by those standards and differed from the later 1940 ISA Series A (0.8 mm pitch for 5 mm, 0.9 mm pitch for 5.5 mm). In Germany the standards (from 1918) were called "DIN" which means "German Industry Standards".
