The kg is defined as being equal to the mass of the International Prototype of the Kilogram (IPK), a block of platinum-iridium alloy manufactured in 1889 and stored at the International Bureau of Weights and Measures in Sèvres, France.
It is the only SI unit that is defined by a physical object rather than a fundamental physical property that can be reproduced in laboratories.
Before approximately the 14th century there were two hundredweights in England, one of 100 pounds, and one of 108 pounds. In 1340, King Edward III changed the value of the stone from 12 pounds to 14 pounds. Since a hundredweight is 8 stones, the 100-pound hundredweight became 112 pounds.