High Quality Content by WIKIPEDIA articles! The metric system was first developed during the French Revolution to replace the existing system of measure. The unit of length in the new system, the metre, was based on the dimensions of the earth, the unit of volume, the litre, was defined as one thousandth of a cubic metre and the unit of mass, the kilogram, was defined as the mass of one litre of water. The system was, in the words of French philosopher Marquis de Condorcet, "for all people for all time".