In the 18th century, most European countries, including Portugal, still used archaic measurement systems, uniform only in theory and with no relationship between the various quantities, which stemmed from reforms dating to the start of the modern era, although with medieval roots.
It would be useful to be able to use a measurement system that could be universal, i.e., used by every country and in all areas of trade and commerce and, at the same time, be sufficiently simple, with easily understood relationships, starting from one single unit.
In 1790, in the context of the French Revolution, the National Constitutional Assembly asked the French Academy of Sciences to introduce a new unit of measurement. They stressed that it should be based on a quantity found in nature, such as the size of the terrestrial globe. They took numerous measurements and calculations, with the participation of various scientists; Delambre and Méchain measured the distance between Dunkirk and Barcelona along the meridian, which was key for calculating the lengh of the arc of meridian from the North Pole to the equator, which amounts to one quarter of the length of each of the Earth’s meridians.
In 1791, the French National Assembly approved the definition of the mètre: one ten-millionth of one quarter of the Earth’s meridian.