The appropriate nominal state is defined in ISO 21771:2006. Measurement technology terminology is explained at the beginning. Deviations on single gears concerning gear axis, profile, helix, pitch and run out as well as allowance factors for tooth thickness test dimensions are considered. Also one- and two-flank roll deviations on test pairs as well as axis and ratio deviations, backlash variation and tooth bearing on gear pairs in the gear box are included.