A variety of units of measurement were used in the various independent Italian states and Italian dependencies of foreign empires up to the unification of Italy in the 19th century. The units to measure length, volume, mass, etc., could differ widely between countries or between towns in a country (e.g. Rome and Ancona), but usually not between a country and its capital. Milan adopted the metric system in 1803, during the Napoleonic wars, albeit reusing names of older units. After the Congress of Vienna, the various Italian states reverted to their original systems of measurements.