The numbers in the system broadly correspond to the US military pay grade system, with OR-x replacing E-x and WO-x replacing W-x. The main difference is in the commissioned officer ranks, where the US system recognises two ranks at OF-1 level (O-1 and O-2), meaning that all O-x numbers after O-1 are one point higher on the US scale than they are on the NATO scale (e.g. a major is OF-3 on the NATO scale and O-4 on the US scale).