Armour with two or more plates spaced a distance apart is called spaced armour. When sloped it reduces the penetrating power of bullets and solid shot as after penetrating each plate they tend to tumble, deflect, deform, or disintegrate; when not sloped it increases the protection offered by the armour because explosive projectiles detonate on it before they reach the inner plates. It has been in use since the First World War, where it was used on the Schneider CA1 and St Chamond tanks. Many WWII German tanks had spaced armour in the form of armoured skirts, Schürzen, to make their thinner side armour more effective against anti-tank fire. Its introduction was to counter Soviet anti-tank units using conventional kinetic penetrator type rounds (AT rifles), not the Bazooka, Panzerfaust, and