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.
Light tanks of the United Kingdom
The biggest change from the Mark IV to the Mark V was the introduction of a three-man crew. The turret now carried the commander and the gunner who was also the radio operator. The increase in the crew size made a lot of difference to the tank's effectiveness and spread the maintenance load. Until then, the commander had to direct the driver, navigate, and operate the gun. If troop commander, he also had to direct the other tanks and their fire.