Tank
With World War II the multi-turreted tanks proved impracticable, and as the single turret on a low hull design became standard, crews became standardized around a crew of four or five. In those tanks with a fifth crew member, usually three were located in the turret (as described above) while the fifth was most often seated in the hull next to the driver, and operated the hull machine gun in addition to acting as a co-driver or radio operator. Well-designed crew stations, giving proper considerations to comfort and ergonomics, are an important factor in the combat effectiveness of a tank, as it limits fatigue and speeds up individual actions.
Tank classification
Tank doctrine in the UK declared that one group of tanks would accompany infantry in a similar role to World War I, while another group of 'cruiser' tanks would then exploit a breakthrough, in a role similar to light cavalry. In the USSR, 1930s tank doctrine specified three groups of tanks: one 'breakthrough' tank in the infantry support role, one tactical breakthrough tank to clear the combat area, and a 'fast tank' for operational maneuver. In Germany, the ideas of Heinz Guderian established the need for unified tank formations, but with a mixture of armaments for differing roles.