Magna Carta demanded one measure of ale for all of England
Among the promises barons extracted from King John in 1215 was a surprisingly practical one. Clause 35 of Magna Carta required a single measure for wine, ale and corn throughout the realm, the London quart, and a standard width for dyed cloth. Fair measures were a political grievance long before they were a scientific tool.
A unit is simply an agreed quantity used as a yardstick: ten metres means ten times one fixed length. Measures were among humanity's earliest inventions, needed for building huts, cutting clothes and bartering grain. The first uniform systems appear to date from the 4th and 3rd millennia BC in Mesopotamia, Egypt and the Indus Valley, and possibly Elam. The Book of Leviticus commands honest weights and measures.
Many old units borrowed from the body. The cubit was a forearm, the pace a stride, and feet and hands speak for themselves, which meant a length could vary from town to town or even person to person. Others came from farming: the furlong, the acre and the carucate were all tied to how much land a team of oxen could work. When different scales needed linking, odd definitions followed, such as an inch expressed in barleycorns.
The push for a universal system began in 1790, when France's National Assembly asked the Academy of Sciences to devise one, and a metric system was adopted in 1791. Global acceptance lagged until 1875, when 17 nations signed the Metre Convention, and the modern International System was adopted in 1954. It rests on seven base units, including the second, metre, kilogram and kelvin, with everything else derived from them. The United States, the only industrialised country not mostly metric, runs both systems; its inch has been defined as exactly 0.0254 metres since a 1959 agreement.
Units follow their own arithmetic. You cannot add metres to seconds, but you can multiply them, and the physicist George Gamow liked to point out that the result is simply a new unit. Speed in metres per second is exactly such a combination, and a newton is shorthand for a kilogram metre per second squared. Specialists keep exotic units too, from the solar mass to the megaton and the electronvolt.
Source: Unit of measurement
