The long ton is the imperial ton or 'weight ton'. It is used in the United Kingdom and several other Commonwealth countries. In their avoirdupois or Imperial system of measurements, it is equal to or of salt water with a density of 64 lb/ft (1.025 g/ml). Many other countries use the metric tonne. It has some limited use in the United States, for measuring the displacement of ships. It was the unit recommended for warships by international agreements between the world wars, for example battleships were limited to 35,000 long tons.