The Mark IV was a British tank of the First World War. Introduced in 1917, it benefitted from significant developments on the first British tank, the intervening designs being small batches used for training. The major improvements were in armour, the re-siting of the fuel tank, and easier transportation. A total of 1,220 were built: 420 'Males', 595 'Females' and 205 Tank Tenders (unarmed vehicles used to carry supplies). As such it was - numerically - the most important tank of the war. The Mark IV was first used in mid 1917 at the Battle of Messines Ridge. They remained in service to the end of the war.