Wikipedia: British heavy tanks were a series of related vehicles developed by the British Army during the First World War. The Mark I was the world's first tank to enter combat. Born of the need to break the domination of trenches, barbed wire and machine guns over the battlefields of the Western Front, it was the first vehicle to be named "tank", a name chosen as an expedient to maintain secrecy and to disguise its true purpose. It was developed to be able to cross trenches, resist small-arms fire, travel over difficult terrain, carry supplies, and to capture fortified enemy positions. It is regarded as successful in many respects, but suffered from many problems owing to its primitive nature. The Mark I entered service in August 1916, and was first used in action on the morning of 15 September 1916 during the Battle of Flers-Courcelette, part of the Somme Offensive. With the exception of the few interim Mark II and Mark III tanks, it was followed by the largely similar Mark IV, which first saw combat in June 1917. The Mark IV was used en masse (about 460 tanks) at the Battle of Cambrai in November 1917. The Mark V, with its much improved transmission, entered service in mid-1918. The unusual rhomboidal shape was to give as long a track run as possible to allow for crossing the wide trenches prevalent on the Western Front battlefields. The heavy tanks were designated "male" or "female" according to the type of armament they carried in the sponsons fitted to their sides. The prototype mounted a 6-pounder (57 mm) cannon and a Hotchkiss machine gun in each. The designers were concerned that the slow-moving vehicles would be vulnerable to attack by enemy infantry, and decided that dedicated anti-personnel weaponry was necessary. Case-shot was not available for the 6-pounder guns, so it was decided to design a new sponson that would instead house two Vickers guns. Tanks fitted with the new design were designated "female", and those with the protruding 6-pounder, "male". (This had the unfortunate effect of making the prototype "Mother" a "male".) The reader is referred to https://en.wikipedia.org/wiki/British_heavy_tanks_of_World_War_I for additional information.