Before the Seventeenth Century every nation used measurement units that had grown from local customs. For example, the English used "three barleycorns, round and dry" as their standard for an inch. Grains of barley varied in size, and so did the inch. As a result, no one could be sure that their measurements of the same thing would be equal.
In 1671, Gabriel Mouton, a French clergyman, proposed a decimal measurement system. We now call this the metric system of measurement.