Mesures usuelles (French pronunciation: [m?zy? yzl], customary measurements) were a French system of measurement introduced by Napoleon I in 1812 to act as compromise between the metric system and traditional measurements. The system was restricted to use in the retail industry and continued in use until 1840, when the laws of measurement from the 1795 and 1799 were reinstituted.
In the five years immediately before the French First Republic introduced the metric system, every effort was made to make the citizens aware of the upcoming changes and to prepare them for it. The administration distributed tens of thousands of educational pamphlets, private enterprise produced educational games, guides, almanacs and conversion aids, and metre standards were built into the walls of prominent buildings around Paris. The introduction was phased by district over the next few years, with Paris being the first district to change. The government also realised that the people would need metre rulers, but they had only provided 25,000 of the 500,000 rulers needed in Paris as late as one month after the metre became the sole legal unit of measure. To compensate, the government introduced incentives for the mass-production of rulers. Paris police reported widespread flouting of the requirement for merchants to use only the metric system. Where the new system was in use, it was abused, with shopkeepers taking the opportunity to round prices up and to give smaller measures.
Napoleon I, the French Emperor, disliked the inconvenience of surrendering the high factorability of traditional measures in the name of decimalisation, and recognized the difficulty of getting it accepted by the populace. Under the décret impérial du 12 février 1812 (imperial decree of 12 February 1812), he introduced a new system of measurement, the mesures usuelles or "customary measures", for use in small retail businesses. However, all government, legal and similar works still had to use the metric system and the metric system continued to be taught at all levels of education.
The prototypes of the metric unit, the kilogram and the metre, enabled an immediate standardization of measurement over the whole country, replacing the varying legal measures in different parts of the country, and even more across the whole of Europe. The new livre (known as the livre métrique) was defined as five hundred grams, and the new toise (toise métrique) was defined as two metres. Products could be sold in shops under the old names and with the old relationships to one another, but with metric-based and slightly changed absolute sizes. This series of measurements was called mesures usuelles.
Napoleon's decree was eventually revoked during the reign of Louis Philippe by the loi du 4 juillet 1837 (law of 4 July 1837), which took effect on 1 January 1840, and reinstated the original metric system. This brought the system of mesures usuelles to a legal end, though the livre remains in some informal use to this day.
The law authorised the following units of measure:
Je me moque des divisions décimales [I don't care about decimal divisions]