The metric system was first developed in France during the French Revolution. A French law passed in 1795 defined five units of measure. Three of these names are still in use today. They are the metre which is the unit of length, the gram which is the unit of mass and the litre which is the unit of volume. Since then many other units of measure have been developed and many definitions changed. The metric system now has units of measurement for energy, power, force, electric current, radioactivity and many others. The most commonly used units of measure in the metric system are listed below.