Effective armor
The thickness a shell actually has to defeat once the angle of the plate is accounted for, which is what penetration is compared against.
Also known as effective thickness, armour effectiveness.
A shell crossing a sloped plate travels a longer path through the steel than the plate is thick. The effective thickness is that path.
effective = thickness / cos(angle from the shell's line of travel)This is why armor is described by shape rather than by numbers. The same plate is worth twice as much when the vehicle is angled toward the shooter and its nominal value when it is caught square, and every armor mechanic in the game, normalization, overmatch, ricochet, is a modifier on this comparison.
Related terms
AnglingTurning a vehicle so incoming shells meet its plates at an angle, multiplying effective armor without changing a single stat.NormalizationThe few degrees a shell turns toward the perpendicular on impact, reducing the effective armor it has to cross.RicochetA shell deflecting off a plate without any penetration check, which happens beyond 70 degrees for most shells and 85 for HEAT.OvermatchThe rule that a shell with a caliber at least three times a plate's thickness cannot ricochet off it, no matter the angle.PenetrationHow much armor a shell can defeat, in millimetres, rolled plus or minus 25 percent on every shot and compared against the effective thickness it hits.