Over time each country, geographical area or in some cases specific guild (trade) defined its own system of measurement units. The usage of these different definitions caused confusion and were barriers to honesty and trade. As early as 1215, the importance of standards is observed in the Magna Carta.
From the 1297 version of the Magna Carta, we see the government taking a role in the definition of units for weights and measures."Art 25 One measure of Wine shall be through our Realm, and one measure of Ale, and one measure of Corn, that is to say, the Quarter of London; and one breadth of dyed Cloth, Russets, and Haberjects, that is to say, two Yards within the lists. And it shall be of Weights as it is of Measures."
In the 19th century, Europe was using many different systems and the metric system was defined to provide a uniform system. In a book titled Vestiges of Pre-Metric Weights and Measures -- Persisting in Metric-System Europe 1926-1927 Arthur E. Kennelly wrote of three types of measurement systems.
Many other useful conversion factors can be found at http://www.wsdot.wa.gov/Metrics/factors.htm
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