The classic material for vehicle armour is a range of special nickel/chrome alloy steels. Steel is a very versatile material and can be produced in a variety of forms but more recent practice favours the use of homogeneous plate which has the same hardness and structure throughout. The production of plate involves a rolling process to bring it to the correct thickness and to induce some desirable metallurgical properties. The armour produced by this method is known as Rolled Homogeneous Armour (RHA) and is used as the current standard of comparison for protection levels offered by other materials. However, different nations have different ways of calculating depth of RHA penetration and care must be taken to compare like with like.