In practice the balancing (mass compensation) of toolholders often takes place in one plane. The tool in such cases has only one flaw in its centre of gravity (centre of gravity eccentricity). The main axis of inertia and the rotational axis are parallel to each other. This condition is called 'static' unbalance. This is the case with almost all normal toolholders operated at rotational speeds of up to 20,000 rpm. The following rule of thumb can be used for assisting a preliminary decision to apply static unbalance correction:  L < 2 x D where L = length of the toolholder and D = cone diameter of the tool being used, e.g., HSK 63 D = 48 mm